@advanced_1000_h1 Advanced Topics @advanced_1001_a Result Sets @advanced_1002_a Large Objects @advanced_1003_a Linked Tables @advanced_1004_a Transaction Isolation @advanced_1005_a Multi-Version Concurrency Control (MVCC) @advanced_1006_a Clustering / High Availability @advanced_1007_a Two Phase Commit @advanced_1008_a Compatibility @advanced_1009_a Run as Windows Service @advanced_1010_a ODBC Driver @advanced_1011_a ACID @advanced_1012_a Durability Problems @advanced_1013_a Using the Recover Tool @advanced_1014_a File Locking Protocols @advanced_1015_a Protection against SQL Injection @advanced_1016_a Security Protocols @advanced_1017_a Universally Unique Identifiers (UUID) @advanced_1018_a Settings Read from System Properties @advanced_1019_a Glossary and Links @advanced_1020_h2 Result Sets @advanced_1021_h3 Limiting the Number of Rows @advanced_1022_p Before the result is returned to the application, all rows are read by the database. Server side cursors are not supported currently. If only the first few rows are interesting for the application, then the result set size should be limited to improve the performance. This can be done using LIMIT in a query (example: SELECT * FROM TEST LIMIT 100), or by using Statement.setMaxRows(max). @advanced_1023_h3 Large Result Sets and External Sorting @advanced_1024_p For result set larger than 1000 rows, the result is buffered to disk. If ORDER BY is used, the sorting is done using an external sort algorithm. In this case, each block of rows is sorted using quick sort, then written to disk; when reading the data, the blocks are merged together. @advanced_1025_h2 Large Objects @advanced_1026_h3 Storing and Reading Large Objects @advanced_1027_p If it is possible that the objects don't fit into memory, then the data type CLOB (for textual data) or BLOB (for binary data) should be used. For these data types, the objects are not fully read into memory, by using streams. To store a BLOB, use PreparedStatement.setBinaryStream. To store a CLOB, use PreparedStatement.setCharacterStream. To read a BLOB, use ResultSet.getBinaryStream, and to read a CLOB, use ResultSet.getCharacterStream. If the client/server mode is used, the BLOB and CLOB data is fully read into memory when accessed. In this case, the size of a BLOB or CLOB is limited by the memory. @advanced_1028_h2 Linked Tables @advanced_1029_p This database supports linked tables, which means tables that don't exist in the current database but are just links to another database. To create such a link, use the CREATE LINKED TABLE statement: @advanced_1030_p It is then possible to access the table in the usual way. There is a restriction when inserting data to this table: When inserting or updating rows into the table, NULL and values that are not set in the insert statement are both inserted as NULL. This may not have the desired effect if a default value in the target table is other than NULL. @advanced_1031_p For each linked table a new connection is opened. This can be a problem for some databases when using many linked tables. For Oracle XE, the maximum number of connection can be increased. Oracle XE needs to be restarted after changing these values: @advanced_1032_h2 Transaction Isolation @advanced_1033_p This database supports the following transaction isolation levels: @advanced_1034_b Read Committed @advanced_1035_li This is the default level. Read locks are released immediately. Higher concurrency is possible when using this level. @advanced_1036_li To enable, execute the SQL statement 'SET LOCK_MODE 3' @advanced_1037_li or append ;LOCK_MODE=3 to the database URL: jdbc:h2:~/test;LOCK_MODE=3 @advanced_1038_b Serializable @advanced_1039_li To enable, execute the SQL statement 'SET LOCK_MODE 1' @advanced_1040_li or append ;LOCK_MODE=1 to the database URL: jdbc:h2:~/test;LOCK_MODE=1 @advanced_1041_b Read Uncommitted @advanced_1042_li This level means that transaction isolation is disabled. @advanced_1043_li To enable, execute the SQL statement 'SET LOCK_MODE 0' @advanced_1044_li or append ;LOCK_MODE=0 to the database URL: jdbc:h2:~/test;LOCK_MODE=0 @advanced_1045_p When using the isolation level 'serializable', dirty reads, non-repeatable reads, and phantom reads are prohibited. @advanced_1046_b Dirty Reads @advanced_1047_li Means a connection can read uncommitted changes made by another connection. @advanced_1048_li Possible with: read uncommitted @advanced_1049_b Non-Repeatable Reads @advanced_1050_li A connection reads a row, another connection changes a row and commits, and the first connection re-reads the same row and gets the new result. @advanced_1051_li Possible with: read uncommitted, read committed @advanced_1052_b Phantom Reads @advanced_1053_li A connection reads a set of rows using a condition, another connection inserts a row that falls in this condition and commits, then the first connection re-reads using the same condition and gets the new row. @advanced_1054_li Possible with: read uncommitted, read committed @advanced_1055_h3 Table Level Locking @advanced_1056_p The database allows multiple concurrent connections to the same database. To make sure all connections only see consistent data, table level locking is used by default. This mechanism does not allow high concurrency, but is very fast. Shared locks and exclusive locks are supported. Before reading from a table, the database tries to add a shared lock to the table (this is only possible if there is no exclusive lock on the object by another connection). If the shared lock is added successfully, the table can be read. It is allowed that other connections also have a shared lock on the same object. If a connection wants to write to a table (update or delete a row), an exclusive lock is required. To get the exclusive lock, other connection must not have any locks on the object. After the connection commits, all locks are released. This database keeps all locks in memory. @advanced_1057_h3 Lock Timeout @advanced_1058_p If a connection cannot get a lock on an object, the connection waits for some amount of time (the lock timeout). During this time, hopefully the connection holding the lock commits and it is then possible to get the lock. If this is not possible because the other connection does not release the lock for some time, the unsuccessful connection will get a lock timeout exception. The lock timeout can be set individually for each connection. @advanced_1059_h2 Multi-Version Concurrency Control (MVCC) @advanced_1060_p The MVCC feature allows higher concurrency than using (table level or row level) locks. When using MVCC in this database, delete, insert and update operations will only issue a shared lock on the table. Table are still locked exclusively when adding or removing columns, when dropping the table, and when using SELECT ... FOR UPDATE. Connections only 'see' committed data, and own changes. That means, if connection A updates a row but doesn't commit this change yet, connection B will see the old value. Only when the change is committed, the new value is visible by other connections (read committed). If multiple connections concurrently try to update the same row, this database fails fast: a concurrent update exception is thrown. @advanced_1061_p To use the MVCC feature, append MVCC=TRUE to the database URL: @advanced_1062_h2 Clustering / High Availability @advanced_1063_p This database supports a simple clustering / high availability mechanism. The architecture is: two database servers run on two different computers, and on both computers is a copy of the same database. If both servers run, each database operation is executed on both computers. If one server fails (power, hardware or network failure), the other server can still continue to work. From this point on, the operations will be executed only on one server until the other server is back up. @advanced_1064_p Clustering can only be used in the server mode (the embedded mode does not support clustering). It is possible to restore the cluster without stopping the server, however it is critical that no other application is changing the data in the first database while the second database is restored, so restoring the cluster is currently a manual process. @advanced_1065_p To initialize the cluster, use the following steps: @advanced_1066_li Create a database @advanced_1067_li Use the CreateCluster tool to copy the database to another location and initialize the clustering. Afterwards, you have two databases containing the same data. @advanced_1068_li Start two servers (one for each copy of the database) @advanced_1069_li You are now ready to connect to the databases with the client application(s) @advanced_1070_h3 Using the CreateCluster Tool @advanced_1071_p To understand how clustering works, please try out the following example. In this example, the two databases reside on the same computer, but usually, the databases will be on different servers. @advanced_1072_li Create two directories: server1 and server2. Each directory will simulate a directory on a computer. @advanced_1073_li Start a TCP server pointing to the first directory. You can do this using the command line: @advanced_1074_li Start a second TCP server pointing to the second directory. This will simulate a server running on a second (redundant) computer. You can do this using the command line: @advanced_1075_li Use the CreateCluster tool to initialize clustering. This will automatically create a new, empty database if it does not exist. Run the tool on the command line: @advanced_1076_li You can now connect to the databases using an application or the H2 Console using the JDBC URL jdbc:h2:tcp://localhost:9101,localhost:9102/test @advanced_1077_li If you stop a server (by killing the process), you will notice that the other machine continues to work, and therefore the database is still accessible. @advanced_1078_li To restore the cluster, you first need to delete the database that failed, then restart the server that was stopped, and re-run the CreateCluster tool. @advanced_1079_h3 Clustering Algorithm and Limitations @advanced_1080_p Read-only queries are only executed against the first cluster node, but all other statements are executed against all nodes. There is currently no load balancing made to avoid problems with transactions. The following functions may yield different results on different cluster nodes and must be executed with care: RANDOM_UUID(), SECURE_RAND(), SESSION_ID(), MEMORY_FREE(), MEMORY_USED(), CSVREAD(), CSVWRITE(), RAND() [when not using a seed]. Those functions should not be used directly in modifying statements (for example INSERT, UPDATE, or MERGE). However, they can be used in read-only statements and the result can then be used for modifying statements. @advanced_1081_h2 Two Phase Commit @advanced_1082_p The two phase commit protocol is supported. 2-phase-commit works as follows: @advanced_1083_li Autocommit needs to be switched off @advanced_1084_li A transaction is started, for example by inserting a row @advanced_1085_li The transaction is marked 'prepared' by executing the SQL statement PREPARE COMMIT transactionName @advanced_1086_li The transaction can now be committed or rolled back @advanced_1087_li If a problem occurs before the transaction was successfully committed or rolled back (for example because a network problem occurred), the transaction is in the state 'in-doubt' @advanced_1088_li When re-connecting to the database, the in-doubt transactions can be listed with SELECT * FROM INFORMATION_SCHEMA.IN_DOUBT @advanced_1089_li Each transaction in this list must now be committed or rolled back by executing COMMIT TRANSACTION transactionName or ROLLBACK TRANSACTION transactionName @advanced_1090_li The database needs to be closed and re-opened to apply the changes @advanced_1091_h2 Compatibility @advanced_1092_p This database is (up to a certain point) compatible to other databases such as HSQLDB, MySQL and PostgreSQL. There are certain areas where H2 is incompatible. @advanced_1093_h3 Transaction Commit when Autocommit is On @advanced_1094_p At this time, this database engine commits a transaction (if autocommit is switched on) just before returning the result. For a query, this means the transaction is committed even before the application scans through the result set, and before the result set is closed. Other database engines may commit the transaction in this case when the result set is closed. @advanced_1095_h3 Keywords / Reserved Words @advanced_1096_p There is a list of keywords that can't be used as identifiers (table names, column names and so on), unless they are quoted (surrounded with double quotes). The list is currently: @advanced_1097_p CURRENT_TIMESTAMP, CURRENT_TIME, CURRENT_DATE, CROSS, DISTINCT, EXCEPT, EXISTS, FROM, FOR, FALSE, FULL, GROUP, HAVING, INNER, INTERSECT, IS, JOIN, LIKE, MINUS, NATURAL, NOT, NULL, ON, ORDER, PRIMARY, ROWNUM, SELECT, SYSDATE, SYSTIME, SYSTIMESTAMP, TODAY, TRUE, UNION, WHERE @advanced_1098_p Certain words of this list are keywords because they are functions that can be used without '()' for compatibility, for example CURRENT_TIMESTAMP. @advanced_1099_h2 Run as Windows Service @advanced_1100_p Using a native wrapper / adapter, Java applications can be run as a Windows Service. There are various tools available to do that. The Java Service Wrapper from Tanuki Software, Inc. ( http://wrapper.tanukisoftware.org ) is included in the installation. Batch files are provided to install, start, stop and uninstall the H2 Database Engine Service. This service contains the TCP Server and the H2 Console web application. The batch files are located in the directory H2/service. @advanced_1101_h3 Install the Service @advanced_1102_p The service needs to be registered as a Windows Service first. To do that, double click on 1_install_service.bat. If successful, a command prompt window will pop up and disappear immediately. If not, a message will appear. @advanced_1103_h3 Start the Service @advanced_1104_p You can start the H2 Database Engine Service using the service manager of Windows, or by double clicking on 2_start_service.bat. Please note that the batch file does not print an error message if the service is not installed. @advanced_1105_h3 Connect to the H2 Console @advanced_1106_p After installing and starting the service, you can connect to the H2 Console application using a browser. Double clicking on 3_start_browser.bat to do that. The default port (8082) is hard coded in the batch file. @advanced_1107_h3 Stop the Service @advanced_1108_p To stop the service, double click on 4_stop_service.bat. Please note that the batch file does not print an error message if the service is not installed or started. @advanced_1109_h3 Uninstall the Service @advanced_1110_p To uninstall the service, double click on 5_uninstall_service.bat. If successful, a command prompt window will pop up and disappear immediately. If not, a message will appear. @advanced_1111_h2 ODBC Driver @advanced_1112_p This database does not come with its own ODBC driver at this time, but it supports the PostgreSQL network protocol. Therefore, the PostgreSQL ODBC driver can be used. Support for the PostgreSQL network protocol is quite new and should be viewed as experimental. It should not be used for production applications. @advanced_1113_p At this time, the PostgreSQL ODBC driver does not work on 64 bit versions of Windows. For more information, see: ODBC Driver on Windows 64 bit @advanced_1114_h3 ODBC Installation @advanced_1115_p First, the ODBC driver must be installed. Any recent PostgreSQL ODBC driver should work, however version 8.2.4 or newer is recommended. The Windows version of the PostgreSQL ODBC driver is available at http://www.postgresql.org/ftp/odbc/versions/msi . @advanced_1116_h3 Starting the Server @advanced_1117_p After installing the ODBC driver, start the H2 Server using the command line: @advanced_1118_p The PG Server (PG for PostgreSQL protocol) is started as well. By default, databases are stored in the current working directory where the server is started. Use -baseDir to save databases in another directory, for example the user home directory: @advanced_1119_p The PG server can be started and stopped from within a Java application as follows: @advanced_1120_p By default, only connections from localhost are allowed. To allow remote connections, use -pgAllowOthers true when starting the server. @advanced_1121_h3 ODBC Configuration @advanced_1122_p After installing the driver, a new Data Source must be added. In Windows, run odbcad32.exe to open the Data Source Administrator. Then click on 'Add...' and select the PostgreSQL Unicode driver. Then click 'Finish'. You will be able to change the connection properties: @advanced_1123_th Property @advanced_1124_th Example @advanced_1125_th Remarks @advanced_1126_td Data Source @advanced_1127_td H2 Test @advanced_1128_td The name of the ODBC Data Source @advanced_1129_td Database @advanced_1130_td test @advanced_1131_td The database name. Only simple names are supported at this time; @advanced_1132_td relative or absolute path are not supported in the database name. @advanced_1133_td By default, the database is stored in the current working directory @advanced_1134_td where the Server is started except when the -baseDir setting is used. @advanced_1135_td The name must be at least 3 characters. @advanced_1136_td Server @advanced_1137_td localhost @advanced_1138_td The server name or IP address. @advanced_1139_td By default, only remote connections are allowed @advanced_1140_td User Name @advanced_1141_td sa @advanced_1142_td The database user name. @advanced_1143_td SSL Mode @advanced_1144_td disabled @advanced_1145_td At this time, SSL is not supported. @advanced_1146_td Port @advanced_1147_td 5435 @advanced_1148_td The port where the PG Server is listening. @advanced_1149_td Password @advanced_1150_td sa @advanced_1151_td The database password. @advanced_1152_p Afterwards, you may use this data source. @advanced_1153_h3 PG Protocol Support Limitations @advanced_1154_p At this time, only a subset of the PostgreSQL network protocol is implemented. Also, there may be compatibility problems on the SQL level, with the catalog, or with text encoding. Problems are fixed as they are found. Currently, statements can not be cancelled when using the PG protocol. @advanced_1155_h3 Security Considerations @advanced_1156_p Currently, the PG Server does not support challenge response or encrypt passwords. This may be a problem if an attacker can listen to the data transferred between the ODBC driver and the server, because the password is readable to the attacker. Also, it is currently not possible to use encrypted SSL connections. Therefore the ODBC driver should not be used where security is important. @advanced_1157_h2 ACID @advanced_1158_p In the database world, ACID stands for: @advanced_1159_li Atomicity: Transactions must be atomic, meaning either all tasks are performed or none. @advanced_1160_li Consistency: All operations must comply with the defined constraints. @advanced_1161_li Isolation: Transactions must be isolated from each other. @advanced_1162_li Durability: Committed transaction will not be lost. @advanced_1163_h3 Atomicity @advanced_1164_p Transactions in this database are always atomic. @advanced_1165_h3 Consistency @advanced_1166_p This database is always in a consistent state. Referential integrity rules are always enforced. @advanced_1167_h3 Isolation @advanced_1168_p For H2, as with most other database systems, the default isolation level is 'read committed'. This provides better performance, but also means that transactions are not completely isolated. H2 supports the transaction isolation levels 'serializable', 'read committed', and 'read uncommitted'. @advanced_1169_h3 Durability @advanced_1170_p This database does not guarantee that all committed transactions survive a power failure. Tests show that all databases sometimes lose transactions on power failure (for details, see below). Where losing transactions is not acceptable, a laptop or UPS (uninterruptible power supply) should be used. If durability is required for all possible cases of hardware failure, clustering should be used, such as the H2 clustering mode. @advanced_1171_h2 Durability Problems @advanced_1172_p Complete durability means all committed transaction survive a power failure. Some databases claim they can guarantee durability, but such claims are wrong. A durability test was run against H2, HSQLDB, PostgreSQL, and Derby. All of those databases sometimes lose committed transactions. The test is included in the H2 download, see org.h2.test.poweroff.Test. @advanced_1173_h3 Ways to (Not) Achieve Durability @advanced_1174_p Making sure that committed transaction are not lost is more complicated than it seems first. To guarantee complete durability, a database must ensure that the log record is on the hard drive before the commit call returns. To do that, databases use different methods. One is to use the 'synchronous write' file access mode. In Java, RandomAccessFile supports the modes "rws" and "rwd": @advanced_1175_li rwd: Every update to the file's content is written synchronously to the underlying storage device. @advanced_1176_li rws: In addition to rwd, every update to the metadata is written synchronously. @advanced_1177_p This feature is used by Derby. A test (org.h2.test.poweroff.TestWrite) with one of those modes achieves around 50 thousand write operations per second. Even when the operating system write buffer is disabled, the write rate is around 50 thousand operations per second. This feature does not force changes to disk because it does not flush all buffers. The test updates the same byte in the file again and again. If the hard drive was able to write at this rate, then the disk would need to make at least 50 thousand revolutions per second, or 3 million RPM (revolutions per minute). There are no such hard drives. The hard drive used for the test is about 7200 RPM, or about 120 revolutions per second. There is an overhead, so the maximum write rate must be lower than that. @advanced_1178_p Buffers can be flushed by calling the function fsync. There are two ways to do that in Java: @advanced_1179_li FileDescriptor.sync(). The documentation says that this forces all system buffers to synchronize with the underlying device. Sync is supposed to return after all in-memory modified copies of buffers associated with this FileDescriptor have been written to the physical medium. @advanced_1180_li FileChannel.force() (since JDK 1.4). This method is supposed to force any updates to this channel's file to be written to the storage device that contains it. @advanced_1181_p By default, MySQL calls fsync for each commit. When using one of those methods, only around 60 write operations per second can be achieved, which is consistent with the RPM rate of the hard drive used. Unfortunately, even when calling FileDescriptor.sync() or FileChannel.force(), data is not always persisted to the hard drive, because most hard drives do not obey fsync(): see 'Your Hard Drive Lies to You' at http://hardware.slashdot.org/article.pl?sid=05/05/13/0529252. In Mac OS X fsync does not flush hard drive buffers: http://lists.apple.com/archives/darwin-dev/2005/Feb/msg00072.html. So the situation is confusing, and tests prove there is a problem. @advanced_1182_p Trying to flush hard drive buffers hard, and if you do the performance is very bad. First you need to make sure that the hard drive actually flushes all buffers. Tests show that this can not be done in a reliable way. Then the maximum number of transactions is around 60 per second. Because of those reasons, the default behavior of H2 is to delay writing committed transactions. @advanced_1183_p In H2, after a power failure, a bit more than one second of committed transactions may be lost. To change the behavior, use SET WRITE_DELAY and CHECKPOINT SYNC. Most other databases support commit delay as well. In the performance comparison, commit delay was used for all databases that support it. @advanced_1184_h3 Running the Durability Test @advanced_1185_p To test the durability / non-durability of this and other databases, you can use the test application in the package org.h2.test.poweroff. Two computers with network connection are required to run this test. One computer just listens, while the test application is run (and power is cut) on the other computer. The computer with the listener application opens a TCP/IP port and listens for an incoming connection. The second computer first connects to the listener, and then created the databases and starts inserting records. The connection is set to 'autocommit', which means after each inserted record a commit is performed automatically. Afterwards, the test computer notifies the listener that this record was inserted successfully. The listener computer displays the last inserted record number every 10 seconds. Now, switch off the power manually, then restart the computer, and run the application again. You will find out that in most cases, none of the databases contains all the records that the listener computer knows about. For details, please consult the source code of the listener and test application. @advanced_1186_h2 Using the Recover Tool @advanced_1187_p The recover tool can be used to extract the contents of a data file, even if the database is corrupted. At this time, it does not extract the content of the log file or large objects (CLOB or BLOB). To run the tool, type on the command line: @advanced_1188_p For each database in the current directory, a text file will be created. This file contains raw insert statement (for the data) and data definition (DDL) statement to recreate the schema of the database. This file cannot be executed directly, as the raw insert statements don't have the correct table names, so the file needs to be pre-processed manually before executing. @advanced_1189_h2 File Locking Protocols @advanced_1190_p Whenever a database is opened, a lock file is created to signal other processes that the database is in use. If database is closed, or if the process that opened the database terminates, this lock file is deleted. @advanced_1191_p In special cases (if the process did not terminate normally, for example because there was a blackout), the lock file is not deleted by the process that created it. That means the existence of the lock file is not a safe protocol for file locking. However, this software uses a challenge-response protocol to protect the database files. There are two methods (algorithms) implemented to provide both security (that is, the same database files cannot be opened by two processes at the same time) and simplicity (that is, the lock file does not need to be deleted manually by the user). The two methods are 'file method' and 'socket methods'. @advanced_1192_h3 File Locking Method 'File' @advanced_1193_p The default method for database file locking is the 'File Method'. The algorithm is: @advanced_1194_li When the lock file does not exist, it is created (using the atomic operation File.createNewFile). Then, the process waits a little bit (20ms) and checks the file again. If the file was changed during this time, the operation is aborted. This protects against a race condition when a process deletes the lock file just after one create it, and a third process creates the file again. It does not occur if there are only two writers. @advanced_1195_li If the file can be created, a random number is inserted together with the locking method ('file'). Afterwards, a watchdog thread is started that checks regularly (every second once by default) if the file was deleted or modified by another (challenger) thread / process. Whenever that occurs, the file is overwritten with the old data. The watchdog thread runs with high priority so that a change to the lock file does not get through undetected even if the system is very busy. However, the watchdog thread does use very little resources (CPU time), because it waits most of the time. Also, the watchdog only reads from the hard disk and does not write to it. @advanced_1196_li If the lock file exists, and it was modified in the 20 ms, the process waits for some time (up to 10 times). If it was still changed, an exception is thrown (database is locked). This is done to eliminate race conditions with many concurrent writers. Afterwards, the file is overwritten with a new version (challenge). After that, the thread waits for 2 seconds. If there is a watchdog thread protecting the file, he will overwrite the change and this process will fail to lock the database. However, if there is no watchdog thread, the lock file will still be as written by this thread. In this case, the file is deleted and atomically created again. The watchdog thread is started in this case and the file is locked. @advanced_1197_p This algorithm is tested with over 100 concurrent threads. In some cases, when there are many concurrent threads trying to lock the database, they block each other (meaning the file cannot be locked by any of them) for some time. However, the file never gets locked by two threads at the same time. However using that many concurrent threads / processes is not the common use case. Generally, an application should throw an error to the user if it cannot open a database, and not try again in a (fast) loop. @advanced_1198_h3 File Locking Method 'Socket' @advanced_1199_p There is a second locking mechanism implemented, but disabled by default. The algorithm is: @advanced_1200_li If the lock file does not exist, it is created. Then a server socket is opened on a defined port, and kept open. The port and IP address of the process that opened the database is written into the lock file. @advanced_1201_li If the lock file exists, and the lock method is 'file', then the software switches to the 'file' method. @advanced_1202_li If the lock file exists, and the lock method is 'socket', then the process checks if the port is in use. If the original process is still running, the port is in use and this process throws an exception (database is in use). If the original process died (for example due to a blackout, or abnormal termination of the virtual machine), then the port was released. The new process deletes the lock file and starts again. @advanced_1203_p This method does not require a watchdog thread actively polling (reading) the same file every second. The problem with this method is, if the file is stored on a network share, two processes (running on different computers) could still open the same database files, if they do not have a direct TCP/IP connection. @advanced_1204_h2 Protection against SQL Injection @advanced_1205_h3 What is SQL Injection @advanced_1206_p This database engine provides a solution for the security vulnerability known as 'SQL Injection'. Here is a short description of what SQL injection means. Some applications build SQL statements with embedded user input such as: @advanced_1207_p If this mechanism is used anywhere in the application, and user input is not correctly filtered or encoded, it is possible for a user to inject SQL functionality or statements by using specially built input such as (in this example) this password: ' OR ''='. In this case the statement becomes: @advanced_1208_p Which is always true no matter what the password stored in the database is. For more information about SQL Injection, see Glossary and Links. @advanced_1209_h3 Disabling Literals @advanced_1210_p SQL Injection is not possible if user input is not directly embedded in SQL statements. A simple solution for the problem above is to use a PreparedStatement: @advanced_1211_p This database provides a way to enforce usage of parameters when passing user input to the database. This is done by disabling embedded literals in SQL statements. To do this, execute the statement: @advanced_1212_p Afterwards, SQL statements with text and number literals are not allowed any more. That means, SQL statement of the form WHERE NAME='abc' or WHERE CustomerId=10 will fail. It is still possible to use PreparedStatements and parameters as described above. Also, it is still possible to generate SQL statements dynamically, and use the Statement API, as long as the SQL statements do not include literals. There is also a second mode where number literals are allowed: SET ALLOW_LITERALS NUMBERS. To allow all literals, execute SET ALLOW_LITERALS ALL (this is the default setting). Literals can only be enabled or disabled by an administrator. @advanced_1213_h3 Using Constants @advanced_1214_p Disabling literals also means disabling hard-coded 'constant' literals. This database supports defining constants using the CREATE CONSTANT command. Constants can be defined only when literals are enabled, but used even when literals are disabled. To avoid name clashes with column names, constants can be defined in other schemas: @advanced_1215_p Even when literals are enabled, it is better to use constants instead of hard-coded number or text literals in queries or views. With constants, typos are found at compile time, the source code is easier to understand and change. @advanced_1216_h3 Using the ZERO() Function @advanced_1217_p It is not required to create a constant for the number 0 as there is already a built-in function ZERO(): @advanced_1218_h2 Security Protocols @advanced_1219_p The following paragraphs document the security protocols used in this database. These descriptions are very technical and only intended for security experts that already know the underlying security primitives. @advanced_1220_h3 User Password Encryption @advanced_1221_p When a user tries to connect to a database, the combination of user name, @, and password hashed using SHA-256, and this hash value is transmitted to the database. This step does not try to an attacker from re-using the value if he is able to listen to the (unencrypted) transmission between the client and the server. But, the passwords are never transmitted as plain text, even when using an unencrypted connection between client and server. That means if a user reuses the same password for different things, this password is still protected up to some point. See also 'RFC 2617 - HTTP Authentication: Basic and Digest Access Authentication' for more information. @advanced_1222_p When a new database or user is created, a new cryptographically secure random salt value is generated. The size of the salt is 64 bit. Using the random salt reduces the risk of an attacker pre-calculating hash values for many different (commonly used) passwords. @advanced_1223_p The combination of user-password hash value (see above) and salt is hashed using SHA-256. The resulting value is stored in the database. When a user tries to connect to the database, the database combines user-password hash value with the stored salt value and calculated the hash value. Other products use multiple iterations (hash the hash value again and again), but this is not done in this product to reduce the risk of denial of service attacks (where the attacker tries to connect with bogus passwords, and the server spends a lot of time calculating the hash value for each password). The reasoning is: if the attacker has access to the hashed passwords, he also has access to the data in plain text, and therefore does not need the password any more. If the data is protected by storing it on another computer and only remotely, then the iteration count is not required at all. @advanced_1224_h3 File Encryption @advanced_1225_p The database files can be encrypted using two different algorithms: AES-128 and XTEA (using 32 rounds). The reasons for supporting XTEA is performance (XTEA is about twice as fast as AES) and to have an alternative algorithm if AES is suddenly broken. @advanced_1226_p When a user tries to connect to an encrypted database, the combination of the word 'file', @, and the file password is hashed using SHA-256. This hash value is transmitted to the server. @advanced_1227_p When a new database file is created, a new cryptographically secure random salt value is generated. The size of the salt is 64 bit. The combination of the file password hash and the salt value is hashed 1024 times using SHA-256. The reason for the iteration is to make it harder for an attacker to calculate hash values for common passwords. @advanced_1228_p The resulting hash value is used as the key for the block cipher algorithm (AES-128 or XTEA with 32 rounds). Then, an initialization vector (IV) key is calculated by hashing the key again using SHA-256. This is to make sure the IV is unknown to the attacker. The reason for using a secret IV is to protect against watermark attacks. @advanced_1229_p Before saving a block of data (each block is 8 bytes long), the following operations are executed: First, the IV is calculated by encrypting the block number with the IV key (using the same block cipher algorithm). This IV is combined with the plain text using XOR. The resulting data is encrypted using the AES-128 or XTEA algorithm. @advanced_1230_p When decrypting, the operation is done in reverse. First, the block is decrypted using the key, and then the IV is calculated combined with the decrypted text using XOR. @advanced_1231_p Therefore, the block cipher modes of operation is CBC (Cipher-block chaining), but each chain is only one block long. The advantage over the ECB (Electronic codebook) mode is that patterns in the data are not revealed, and the advantage over multi block CBC is that flipped cipher text bits are not propagated to flipped plaintext bits in the next block. @advanced_1232_p Database encryption is meant for securing the database while it is not in use (stolen laptop and so on). It is not meant for cases where the attacker has access to files while the database is in use. When he has write access, he can for example replace pieces of files with pieces of older versions and manipulate data like this. @advanced_1233_p File encryption slows down the performance of the database engine. Compared to unencrypted mode, database operations take about 2.2 times longer when using XTEA, and 2.5 times longer using AES (embedded mode). @advanced_1234_h3 SSL/TLS Connections @advanced_1235_p Remote SSL/TLS connections are supported using the Java Secure Socket Extension (SSLServerSocket / SSLSocket). By default, anonymous SSL is enabled. The default cipher suite is SSL_DH_anon_WITH_RC4_128_MD5 . @advanced_1236_h3 HTTPS Connections @advanced_1237_p The web server supports HTTP and HTTPS connections using SSLServerSocket. There is a default self-certified certificate to support an easy starting point, but custom certificates are supported as well. @advanced_1238_h2 Universally Unique Identifiers (UUID) @advanced_1239_p This database supports the UUIDs. Also supported is a function to create new UUIDs using a cryptographically strong pseudo random number generator. With random UUIDs, the chance of two having the same value can be calculated using the probability theory. See also 'Birthday Paradox'. Standardized randomly generated UUIDs have 122 random bits. 4 bits are used for the version (Randomly generated UUID), and 2 bits for the variant (Leach-Salz). This database supports generating such UUIDs using the built-in function RANDOM_UUID(). Here is a small program to estimate the probability of having two identical UUIDs after generating a number of values: @advanced_1240_p Some values are: @advanced_1241_p To help non-mathematicians understand what those numbers mean, here a comparison: One's annual risk of being hit by a meteorite is estimated to be one chance in 17 billion, that means the probability is about 0.000'000'000'06. @advanced_1242_h2 Settings Read from System Properties @advanced_1243_p Some settings of the database can be set on the command line using -DpropertyName=value. It is usually not required to change those settings manually. The settings are case sensitive. Example: @advanced_1244_p The current value of the settings can be read in the table INFORMATION_SCHEMA.SETTINGS @advanced_1245_th Setting @advanced_1246_th Default @advanced_1247_th Description @advanced_1248_td h2.check @advanced_1249_td true @advanced_1250_td Assertions in the database engine @advanced_1251_td h2.check2 @advanced_1252_td false @advanced_1253_td Additional assertions @advanced_1254_td h2.clientTraceDirectory @advanced_1255_td trace.db/ @advanced_1256_td Directory where the trace files of the JDBC client are stored (only for client / server) @advanced_1257_td h2.emergencySpaceInitial @advanced_1258_td 1048576 @advanced_1259_td Size of 'reserve' file to detect disk full problems early @advanced_1260_td h2.emergencySpaceMin @advanced_1261_td 131072 @advanced_1262_td Minimum size of 'reserve' file @advanced_1263_td h2.lobCloseBetweenReads @advanced_1264_td false @advanced_1265_td Close LOB files between read operations @advanced_1266_td h2.lobFilesInDirectories @advanced_1267_td false @advanced_1268_td Store LOB files in subdirectories @advanced_1269_td h2.lobFilesPerDirectory @advanced_1270_td 256 @advanced_1271_td Maximum number of LOB files per directory @advanced_1272_td h2.logAllErrors @advanced_1273_td false @advanced_1274_td Write stack traces of any kind of error to a file @advanced_1275_td h2.logAllErrorsFile @advanced_1276_td h2errors.txt @advanced_1277_td File name to log errors @advanced_1278_td h2.maxFileRetry @advanced_1279_td 16 @advanced_1280_td Number of times to retry file delete and rename @advanced_1281_td h2.objectCache @advanced_1282_td true @advanced_1283_td Cache commonly used objects (integers, strings) @advanced_1284_td h2.objectCacheMaxPerElementSize @advanced_1285_td 4096 @advanced_1286_td Maximum size of an object in the cache @advanced_1287_td h2.objectCacheSize @advanced_1288_td 1024 @advanced_1289_td Size of object cache @advanced_1290_td h2.optimizeEvaluatableSubqueries @advanced_1291_td true @advanced_1292_td Optimize subqueries that are not dependent on the outer query @advanced_1293_td h2.optimizeIn @advanced_1294_td true @advanced_1295_td Optimize IN(...) comparisons @advanced_1296_td h2.optimizeMinMax @advanced_1297_td true @advanced_1298_td Optimize MIN and MAX aggregate functions @advanced_1299_td h2.optimizeSubqueryCache @advanced_1300_td true @advanced_1301_td Cache subquery results @advanced_1302_td h2.overflowExceptions @advanced_1303_td true @advanced_1304_td Throw an exception on integer overflows @advanced_1305_td h2.recompileAlways @advanced_1306_td false @advanced_1307_td Always recompile prepared statements @advanced_1308_td h2.redoBufferSize @advanced_1309_td 262144 @advanced_1310_td Size of the redo buffer (used at startup when recovering) @advanced_1311_td h2.runFinalize @advanced_1312_td true @advanced_1313_td Run finalizers to detect unclosed connections @advanced_1314_td h2.scriptDirectory @advanced_1315_td Relative or absolute directory where the script files are stored to or read from @advanced_1316_td h2.serverCachedObjects @advanced_1317_td 64 @advanced_1318_td TCP Server: number of cached objects per session @advanced_1319_td h2.serverSmallResultSetSize @advanced_1320_td 100 @advanced_1321_td TCP Server: result sets below this size are sent in one block @advanced_1322_h2 Glossary and Links @advanced_1323_th Term @advanced_1324_th Description @advanced_1325_td AES-128 @advanced_1326_td A block encryption algorithm. See also: Wikipedia: AES @advanced_1327_td Birthday Paradox @advanced_1328_td Describes the higher than expected probability that two persons in a room have the same birthday. Also valid for randomly generated UUIDs. See also: Wikipedia: Birthday Paradox @advanced_1329_td Digest @advanced_1330_td Protocol to protect a password (but not to protect data). See also: RFC 2617: HTTP Digest Access Authentication @advanced_1331_td GCJ @advanced_1332_td GNU Compiler for Java. http://gcc.gnu.org/java/ and http://nativej.mtsystems.ch/ (not free any more) @advanced_1333_td HTTPS @advanced_1334_td A protocol to provide security to HTTP connections. See also: RFC 2818: HTTP Over TLS @advanced_1335_td Modes of Operation @advanced_1336_a Wikipedia: Block cipher modes of operation @advanced_1337_td Salt @advanced_1338_td Random number to increase the security of passwords. See also: Wikipedia: Key derivation function @advanced_1339_td SHA-256 @advanced_1340_td A cryptographic one-way hash function. See also: Wikipedia: SHA hash functions @advanced_1341_td SQL Injection @advanced_1342_td A security vulnerability where an application generates SQL statements with embedded user input. See also: Wikipedia: SQL Injection @advanced_1343_td Watermark Attack @advanced_1344_td Security problem of certain encryption programs where the existence of certain data can be proven without decrypting. For more information, search in the internet for 'watermark attack cryptoloop' @advanced_1345_td SSL/TLS @advanced_1346_td Secure Sockets Layer / Transport Layer Security. See also: Java Secure Socket Extension (JSSE) @advanced_1347_td XTEA @advanced_1348_td A block encryption algorithm. See also: Wikipedia: XTEA @build_1000_h1 Build @build_1001_a Portability @build_1002_a Environment @build_1003_a Building the Software @build_1004_a Using Maven 2 @build_1005_a Translating @build_1006_h2 Portability @build_1007_p This database is written in Java and therefore works on many platforms. It can also be compiled to a native executable using GCJ. @build_1008_h2 Environment @build_1009_p A Java Runtime Environment (JRE) version 1.4 or higher is required to run this database. @build_1010_p To build the database executables, the following software stack was used. Newer version or compatible software works too. @build_1011_li Windows XP @build_1012_li Sun JDK Version 1.4 or 1.5 @build_1013_li Apache Ant Version 1.6.5 @build_1014_li Mozilla Firefox 1.5 @build_1015_li Eclipse Version 3.2.2 @build_1016_li YourKit Java Profiler @build_1017_h2 Building the Software @build_1018_p On the command line, go to the directory src and execute the following command: @build_1019_p You will get a list of targets. If you want to build the jar files, execute: @build_1020_p To create a jar file with the JDBC API and the classes required to connect to a server only, use the target jarClient: @build_1021_p The other targets may be used as well. @build_1022_h2 Using Maven 2 @build_1023_h3 Using a Central Repository @build_1024_p You can include the database in your Maven 2 project as a dependency. Example: @build_1025_p New versions of this database are first uploaded to http://hsql.sourceforge.net/m2-repo/ and then automatically synchronized with the main maven repository; however after a new release it may take a few hours before they are available there. @build_1026_h3 Using Snapshot Version @build_1027_p To build a 'snapshot' H2 .jar file and upload it the to the local Maven 2 repository, execute the following command: @build_1028_p Afterwards, you can include the database in your Maven 2 project as a dependency: @build_1029_h2 Translating @build_1030_p The translation of this software is split into the following parts: @build_1031_li H2 Console: src/main/org/h2/server/web/res/_text_*.properties @build_1032_li Error messages: src/main/org/h2/res/_messages_*.properties @build_1033_li Web site: src/docsrc/text/_docs_*.utf8.txt @build_1034_p The conversion between UTF-8 and Java encoding (using the \u syntax), as well as the HTML entities (&#..;) is automated by running the tool PropertiesToUTF8. The web site translation is automated as well, using ant docs . @download_1000_h1 Downloads @download_1001_h3 Version 1.0.63 (2007-12-02, Current) @download_1002_a Windows Installer @download_1003_a Platform-Independent Zip @download_1004_h3 Download Mirror @download_1005_a Platform-Independent Zip @download_1006_h3 Subversion Source Repository @download_1007_a Google Code @download_1008_p For details about changes, see the Change Log . @faq_1000_h1 Frequently Asked Questions @faq_1001_a Are there any known bugs? When is the next release? @faq_1002_a Is this Database Engine Open Source? @faq_1003_a My query is slow @faq_1004_a How to Create a New Database? @faq_1005_a How to Connect to a Database? @faq_1006_a Where are the Database Files Stored? @faq_1007_a What is the Size Limit (maximum size) of a Database? @faq_1008_a Is it Reliable? @faq_1009_a Is the GCJ version stable? Faster? @faq_1010_a How to Translate this Project? @faq_1011_h3 Are there any known bugs? When is the next release? @faq_1012_p Usually, bugs get fixes as they are found. There is a release every few weeks. Here is the list of known and confirmed issues: @faq_1013_li Some problems have been found with right outer join. Internally, it is converted to left outer join, which does not always produce the same results as other databases when used in combination with other joins. @faq_1014_h3 Is this Database Engine Open Source? @faq_1015_p Yes. It is free to use and distribute, and the source code is included. See also under license. @faq_1016_h3 My query is slow @faq_1017_p Slow SELECT (or DELETE, UPDATE, MERGE) statement can have multiple reasons. Follow this checklist: @faq_1018_li Run ANALYSE (see documentation for details). @faq_1019_li Run the query with EXPLAIN and check if indexes are used (see documentation for details). @faq_1020_li If required, create additional indexes and try again using ANALYZE and EXPLAIN. @faq_1021_li If it doesn't help please report the problem. @faq_1022_h3 How to Create a New Database? @faq_1023_p By default, a new database is automatically created if it does not yet exist. @faq_1024_h3 How to Connect to a Database? @faq_1025_p The database driver is org.h2.Driver , and the database URL starts with jdbc:h2: . To connect to a database using JDBC, use the following code: @faq_1026_h3 Where are the Database Files Stored? @faq_1027_p When using database URLs like jdbc:h2:~/test, the database is stored in the user directory. For Windows, this is usually C:\Documents and Settings\<userName>. If the base directory is not set (as in jdbc:h2:test), the database files are stored in the directory where the application is started (the current working directory). When using the H2 Console application from the start menu, this is [Installation Directory]/bin. The base directory can be set in the database URL. A fixed or relative path can be used. When using the URL jdbc:h2:file:data/sample, the database is stored in the directory data (relative to the current working directory). The directory must exist. It is also possible to use the fully qualified directory (and for Windows, drive) name. Example: jdbc:h2:file:C:/data/test @faq_1028_h3 What is the Size Limit (maximum size) of a Database? @faq_1029_p The theoretical limit is currently 256 GB for the data. This number is excluding BLOB and CLOB data: Every CLOB or BLOB can be up to 256 GB as well. The size limit of the index data is 256 GB as well. @faq_1030_p The maximum file size for FAT or FAT32 file systems is 4 GB. So if you use FAT or FAT32, the limit is 4 GB for the data. @faq_1031_h3 Is it Reliable? @faq_1032_p That is not easy to say. It is still a quite new product. A lot of tests have been written, and the code coverage of these tests is very high. Randomized stress tests are run regularly. But as this is a relatively new product, there are probably some problems that have not yet been found. Areas that are not 100% tested: @faq_1033_li Platforms other than Windows XP and the Sun JVM 1.4 and 1.5 @faq_1034_li Data types BLOB, CLOB, VARCHAR_IGNORECASE, OTHER @faq_1035_li Cluster mode, 2-Phase Commit, Savepoints @faq_1036_li Server mode (well tested, but not as well as Embedded mode) @faq_1037_li Multi-Threading and using multiple connections @faq_1038_li Updatable result sets @faq_1039_li Referential integrity and check constraints, Triggers @faq_1040_li ALTER TABLE statements, Views, Linked Tables, Schema, UNION @faq_1041_li Not all built-in functions are completely tested @faq_1042_li The Optimizer may not always select the best plan @faq_1043_li 24/7 operation and large databases (500 MB and up) @faq_1044_li Wide indexes with large VARCHAR or VARBINARY columns and / or with a lot of columns @faq_1045_p Areas considered Experimental: @faq_1046_li ODBC driver and the GCJ native version on Windows @faq_1047_li Linear Hash Index @faq_1048_li Compatibility modes for other databases (only some features are implemented) @faq_1049_li The ARRAY data type and related functionality. @faq_1050_h3 Is the GCJ version stable? Faster? @faq_1051_p The GCJ version is not as stable as the Java version. When running the regression test with the GCJ version, sometimes the application just stops at what seems to be a random point without error message. Currently, the GCJ version is also slower than when using the Sun VM. However, the startup of the GCJ version is faster than when using a VM. @faq_1052_h3 How to Translate this Project? @faq_1053_p For more information, see Build/Translating . @features_1000_h1 Features @features_1001_a Feature List @features_1002_a Limitations @features_1003_a Comparison to Other Database Engines @features_1004_a H2 in Use @features_1005_a Connection Modes @features_1006_a Database URL Overview @features_1007_a Memory-Only Databases @features_1008_a Connecting to a Database with File Encryption @features_1009_a Database File Locking @features_1010_a Opening a Database Only if it Already Exists @features_1011_a Closing the Database @features_1012_a Log Index Changes @features_1013_a Custom File Access Mode @features_1014_a Multiple Connections @features_1015_a Database File Layout @features_1016_a Logging and Recovery @features_1017_a Compatibility @features_1018_a Using the Trace Options @features_1019_a Read Only Databases @features_1020_a Read Only Databases in Zip or Jar File @features_1021_a Binary and Text Storage Formats @features_1022_a Graceful Handling of Low Disk Space Situations @features_1023_a Computed Columns / Function Based Index @features_1024_a Multi-Dimensional Indexes @features_1025_a Using Passwords @features_1026_a User Defined Functions and Stored Procedures @features_1027_a Triggers @features_1028_a Compacting a Database @features_1029_a Cache Settings @features_1030_a Why Java @features_1031_h2 Feature List @features_1032_h3 Main Features @features_1033_li Very fast database engine @features_1034_li Free, with source code @features_1035_li Written in Java @features_1036_li Supports standard SQL, JDBC API @features_1037_li Embedded and Server mode, Clustering support @features_1038_li Strong security features @features_1039_li Experimental native version (GCJ) and ODBC drivers @features_1040_h3 Additional Features @features_1041_li Disk based or in-memory databases and tables, read-only database support, temporary tables @features_1042_li Transaction support (read committed and serializable transaction isolation), 2-phase-commit @features_1043_li Multiple connections, table level locking @features_1044_li Cost based optimizer, using a genetic algorithm for complex queries, zero-administration @features_1045_li Scrollable and updatable result set support, large result set, external result sorting, functions can return a result set @features_1046_li Encrypted database (AES or XTEA), SHA-256 password encryption, encryption functions, SSL @features_1047_h3 SQL Support @features_1048_li Support for multiple schemas, information schema @features_1049_li Referential integrity / foreign key constraints with cascade, check constraints @features_1050_li Inner and outer joins, subqueries, read only views and inline views @features_1051_li Triggers and Java functions / stored procedures @features_1052_li Many built-in functions, including XML and lossless data compression @features_1053_li Wide range of data types including large objects (BLOB/CLOB) and arrays @features_1054_li Sequence and autoincrement columns, computed columns (can be used for function based indexes) @features_1055_li ORDER BY, GROUP BY, HAVING, UNION, LIMIT, TOP @features_1056_li Collation support, users, roles @features_1057_li Compatibility modes for HSQLDB, MySQL and PostgreSQL @features_1058_h3 Security Features @features_1059_li Includes a solution for the SQL injection problem @features_1060_li User password authenticated uses SHA-256 and salt @features_1061_li User passwords are never transmitted in plain text over the network (even when using insecure connections) @features_1062_li All database files (including script files that can be used to backup data) can be encrypted using AES-256 and XTEA encryption algorithms @features_1063_li The remote JDBC driver supports TCP/IP connections over SSL/TLS @features_1064_li The built-in web server supports connections over SSL/TLS @features_1065_li Passwords can be sent to the database using char arrays instead of Strings @features_1066_h3 Other Features and Tools @features_1067_li Small footprint (smaller than 1 MB), low memory requirements @features_1068_li Multiple index types (b-tree, tree, hash, linear hash) @features_1069_li Support for multi-dimensional indexes @features_1070_li CSV (comma separated values) file support @features_1071_li Support for linked tables, and a built-in virtual 'range' table @features_1072_li EXPLAIN PLAN support, sophisticated trace options @features_1073_li Database closing can be delayed or disabled to improve the performance @features_1074_li Web-based Console application (English, German, partially French and Spanish) with autocomplete @features_1075_li The database can generate SQL script files @features_1076_li Contains a recovery tool that can dump the contents of the data file @features_1077_li Automatic re-compilation of prepared statements @features_1078_li Uses a small number of database files, binary and text storage formats, graceful handling of low disk space situations @features_1079_li Uses a checksum for each record and log entry for data integrity @features_1080_li Well tested (high code coverage, randomized stress tests) @features_1081_h2 Limitations @features_1082_p For the list of limitations, please have a look at the road map page at: http://groups.google.com/group/h2-database/web/roadmap @features_1083_h2 Comparison to Other Database Engines @features_1084_th Feature @features_1085_th H2 @features_1086_th Derby @features_1087_th HSQLDB @features_1088_th MySQL @features_1089_th PostgreSQL @features_1090_td Embedded Mode (Java) @features_1091_td Yes @features_1092_td Yes @features_1093_td Yes @features_1094_td No @features_1095_td No @features_1096_td Pure Java @features_1097_td Yes @features_1098_td Yes @features_1099_td Yes @features_1100_td No @features_1101_td No @features_1102_td Performance (Embedded) @features_1103_td Fast @features_1104_td Slow @features_1105_td Fast @features_1106_td N/A @features_1107_td N/A @features_1108_td Transaction Isolation @features_1109_td Yes @features_1110_td Yes @features_1111_td No @features_1112_td Yes @features_1113_td Yes @features_1114_td Cost Based Optimizer @features_1115_td Yes @features_1116_td Yes @features_1117_td No @features_1118_td Yes @features_1119_td Yes @features_1120_td Clustering @features_1121_td Yes @features_1122_td No @features_1123_td No @features_1124_td Yes @features_1125_td Yes @features_1126_td Encrypted Database @features_1127_td Yes @features_1128_td Yes @features_1129_td No @features_1130_td No @features_1131_td No @features_1132_td Files per Database @features_1133_td Few @features_1134_td Many @features_1135_td Few @features_1136_td Many @features_1137_td Many @features_1138_td Footprint (jar/dll size) @features_1139_td ~ 1 MB @features_1140_td ~ 2 MB @features_1141_td ~ 600 KB @features_1142_td ~ 4 MB @features_1143_td ~ 6 MB @features_1144_h3 Derby and HSQLDB @features_1145_p After an unexpected process termination (for example power failure), H2 can recover safely and automatically without any user interaction. For Derby and HSQLDB, there are some manual steps required ('Another instance of Derby may have already booted the database' / 'The database is already in use by another process'). @features_1146_h3 DaffodilDb and One$Db @features_1147_p It looks like the development of this database has stopped. The last release was February 2006. @features_1148_h3 McKoi @features_1149_p It looks like the development of this database has stopped. The last release was August 2004 @features_1150_h2 H2 in Use @features_1151_p For a list of applications that work with or use H2, see: http://groups.google.com/group/h2-database/web/h2-in-use @features_1152_h2 Connection Modes @features_1153_p The following connection modes are supported: @features_1154_li Local connections using JDBC (embedded) @features_1155_li Remote connections using JDBC over TCP/IP (client/server) @features_1156_li Remote connections using ODBC over TCP/IP (client/server) @features_1157_li In-Memory databases (private and shared) @features_1158_h2 Database URL Overview @features_1159_p This database does support multiple connection modes and features when connecting to a database. This is achieved using different database URLs. The settings in the URLs are not case sensitive. @features_1160_th Topic @features_1161_th URL Format and Examples @features_1162_td Embedded (local) connection @features_1163_td jdbc:h2:[file:][<path>]<databaseName> @features_1164_td jdbc:h2:~/test @features_1165_td jdbc:h2:file:/data/sample @features_1166_td jdbc:h2:file:C:/data/sample (Windows only) @features_1167_td In-Memory (private) @features_1168_td jdbc:h2:mem: @features_1169_td In-Memory (named) @features_1170_td jdbc:h2:mem:<databaseName> @features_1171_td jdbc:h2:mem:test_mem @features_1172_td Remote using TCP/IP @features_1173_td jdbc:h2:tcp://<server>[:<port>]/<databaseName> @features_1174_td jdbc:h2:tcp://localhost/test @features_1175_td jdbc:h2:tcp://dbserv:8084/sample @features_1176_td Remote using SSL/TLS @features_1177_td jdbc:h2:ssl://<server>[:<port>]/<databaseName> @features_1178_td jdbc:h2:ssl://secureserv:8085/sample; @features_1179_td Using Encrypted Files @features_1180_td jdbc:h2:<url>;CIPHER=[AES|XTEA] @features_1181_td jdbc:h2:ssl://secureserv/testdb;CIPHER=AES @features_1182_td jdbc:h2:file:~/secure;CIPHER=XTEA @features_1183_td File Locking Methods @features_1184_td jdbc:h2:<url>;FILE_LOCK={NO|FILE|SOCKET} @features_1185_td jdbc:h2:file:~/quickAndDirty;FILE_LOCK=NO @features_1186_td jdbc:h2:file:~/private;CIPHER=XTEA;FILE_LOCK=SOCKET @features_1187_td Only Open if it Already Exists @features_1188_td jdbc:h2:<url>;IFEXISTS=TRUE @features_1189_td jdbc:h2:file:~/sample;IFEXISTS=TRUE @features_1190_td Don't Close the Database when the VM Exits @features_1191_td jdbc:h2:<url>;DB_CLOSE_ON_EXIT=FALSE @features_1192_td User Name and/or Password @features_1193_td jdbc:h2:<url>[;USER=<username>][;PASSWORD=<value>] @features_1194_td jdbc:h2:file:~/sample;USER=sa;PASSWORD=123 @features_1195_td Log Index Changes @features_1196_td jdbc:h2:<url>;LOG=2 @features_1197_td jdbc:h2:file:~/sample;LOG=2 @features_1198_td Debug Trace Settings @features_1199_td jdbc:h2:<url>;TRACE_LEVEL_FILE=<level 0..3> @features_1200_td jdbc:h2:file:~/sample;TRACE_LEVEL_FILE=3 @features_1201_td Ignore Unknown Settings @features_1202_td jdbc:h2:<url>;IGNORE_UNKNOWN_SETTINGS=TRUE @features_1203_td Custom File Access Mode @features_1204_td jdbc:h2:<url>;ACCESS_MODE_LOG=rws;ACCESS_MODE_DATA=rws @features_1205_td In-Memory (private) @features_1206_td jdbc:h2:mem: @features_1207_td Database in or Zip File @features_1208_td jdbc:h2:zip:<zipFileName>!/<databaseName> @features_1209_td jdbc:h2:zip:db.zip!/test @features_1210_td Changing Other Settings @features_1211_td jdbc:h2:<url>;<setting>=<value>[;<setting>=<value>...] @features_1212_td jdbc:h2:file:~/sample;TRACE_LEVEL_SYSTEM_OUT=3 @features_1213_h3 Connecting to an Embedded (Local) Database @features_1214_p The database URL for connecting to a local database is jdbc:h2:[file:][<path>]<databaseName> . The prefix file: is optional. If no or only a relative path is used, then the current working directory is used as a starting point. The case sensitivity of the path and database name depend on the operating system, however it is suggested to use lowercase letters only. The database name must be at least three characters long (a limitation of File.createTempFile). To point to the user home directory, use ~/, as in: jdbc:h2:~/test. @features_1215_h2 Memory-Only Databases @features_1216_p For certain use cases (for example: rapid prototyping, testing, high performance operations, read-only databases), it may not be required to persist (changes to) the data at all. This database supports the memory-only mode, where the data is not persisted. @features_1217_p In some cases, only one connection to a memory-only database is required. This means the database to be opened is private. In this case, the database URL is jdbc:h2:mem: Opening two connections within the same virtual machine means opening two different (private) databases. @features_1218_p Sometimes multiple connections to the same memory-only database are required. In this case, the database URL must include a name. Example: jdbc:h2:mem:db1 . Accessing the same database in this way only works within the same virtual machine and class loader environment. @features_1219_p It is also possible to access a memory-only database remotely (or from multiple processes in the same machine) using TCP/IP or SSL/TLS. An example database URL is: jdbc:h2:tcp://localhost/mem:db1 (using private database remotely is also possible). @features_1220_p By default, when the last connection to a in-memory database is closed, the contents are lost. This can be disabled by adding ;DB_CLOSE_DELAY=-1 to the database URL. That means to keep the contents of an in-memory database as long as the virtual machine is alive, use jdbc:h2:mem:test;DB_CLOSE_DELAY=-1 @features_1221_h2 Connecting to a Database with File Encryption @features_1222_p To use file encryption, it is required to specify the encryption algorithm (the 'cipher') and the file password. The algorithm needs to be specified using the connection parameter. Two algorithms are supported: XTEA and AES. The file password is specified in the password field, before the user password. A single space needs to be added between the file password and the user password; the file password itself may not contain spaces. File passwords (as well as user passwords) are case sensitive. Here is an example to connect to a password encrypted database: @features_1223_h2 Database File Locking @features_1224_p Whenever a database is opened, a lock file is created to signal other processes that the database is in use. If database is closed, or if the process that opened the database terminates, this lock file is deleted. @features_1225_p The following file locking methods are implemented: @features_1226_li The default method is 'file' and uses a watchdog thread to protect the database file. The watchdog reads the lock file each second. @features_1227_li The second method is 'socket' and opens a server socket. The socket method does not require reading the lock file every second. The socket method should only be used if the database files are only accessed by the one (and always the same) computer. @features_1228_li It is also possible to open the database without file locking; in this case it is up to the application to protect the database files. @features_1229_p To open the database with a different file locking method, use the parameter 'FILE_LOCK'. The following code opens the database with the 'socket' locking method: @features_1230_p The following code forces the database to not create a lock file at all. Please note that this is unsafe as another process is able to open the same database, possibly leading to data corruption: @features_1231_p For more information about the algorithms please see in Advanced Topics under File Locking Protocol. @features_1232_h2 Opening a Database Only if it Already Exists @features_1233_p By default, when an application calls DriverManager.getConnection(url,...) and the database specified in the URL does not yet exist, a new (empty) database is created. In some situations, it is better to restrict creating new database, and only open the database if it already exists. This can be done by adding ;ifexists=true to the URL. In this case, if the database does not already exist, an exception is thrown when trying to connect. The connection only succeeds when the database already exists. The complete URL may look like this: @features_1234_h2 Closing the Database @features_1235_h3 Delayed Database Closing @features_1236_p Usually, the database is closed when the last connection to it is closed. In some situations this slows down the application, for example when it is not possible leave the connection open. The automatic closing of the database can be delayed or disabled with the SQL statement SET DB_CLOSE_DELAY <seconds>. The seconds specifies the number of seconds to keep a database open after the last connection to it was closed. For example the following statement will keep the database open for 10 seconds: @features_1237_p The value -1 means the database is never closed automatically. The value 0 is the default and means the database is closed when the last connection is closed. This setting is persistent and can be set by an administrator only. It is possible to set the value in the database URL: jdbc:h2:~/test;DB_CLOSE_DELAY=10 . @features_1238_h3 Don't Close the Database when the VM Exits @features_1239_p By default, a database is closed when the last connection is closed. However, if it is never closed, the database is closed when the virtual machine exits normally. This is done using a shutdown hook. In some situations, the database should not be closed in this case, for example because the database is still used at virtual machine shutdown (to store the shutdown process in the database for example). For those cases, the automatic closing of the database can be disabled in the database URL. The first connection (the one that is opening the database) needs to set the option in the database URL (it is not possible to change the setting afterwards). The database URL to disable database closing on exit is: @features_1240_h2 Log Index Changes @features_1241_p Usually, changes to the index file are not logged for performance. If the index file is corrupt or missing when opening a database, it is re-created from the data. The index file can get corrupt when the database is not shut down correctly, because of power failure or abnormal program termination. In some situations, for example when using very large databases (over a few hundred MB), re-creating the index file takes very long. In these situations it may be better to log changes to the index file, so that recovery from a corrupted index file is fast. To enable log index changes, add LOG=2 to the URL, as in jdbc:h2:~/test;LOG=2 This setting should be specified when connecting. The update performance of the database will be reduced when using this option. @features_1242_h3 Ignore Unknown Settings @features_1243_p Some applications (for example OpenOffice.org Base) pass some additional parameters when connecting to the database. Why those parameters are passed is unknown. The parameters PREFERDOSLIKELINEENDS and IGNOREDRIVERPRIVILEGES are such examples, they are simply ignored to improve the compatibility with OpenOffice.org. If an application passes other parameters when connecting to the database, usually the database throws an exception saying the parameter is not supported. It is possible to ignored such parameters by adding ;IGNORE_UNKNOWN_SETTINGS=TRUE to the database URL. @features_1244_h3 Changing Other Settings when Opening a Connection @features_1245_p In addition to the settings already described (cipher, file_lock, ifexists, user, password), other database settings can be passed in the database URL. Adding setting=value at the end of an URL is the same as executing the statement SET setting value just after connecting. For a list of settings supported by this database please see the SQL grammar documentation. @features_1246_h2 Custom File Access Mode @features_1247_p Usually, the database opens log, data and index files with the access mode 'rw', meaning read-write (except for read only databases, where the mode 'r' is used). Also supported are 'rws' and 'rwd'. The access mode used for log files is set via ACCESS_MODE_LOG; for data and index files use ACCESS_MODE_DATA. These settings must be specified in the database URL: @features_1248_p For more information see Durability Problems . On many operating systems the access mode 'rws' does not guarantee that the data is written to the disk. @features_1249_h2 Multiple Connections @features_1250_h3 Opening Multiple Databases at the Same Time @features_1251_p An application can open multiple databases at the same time, including multiple connections to the same database. The number of open database is only limited by the memory available. @features_1252_h3 Multiple Connections to the Same Database: Client/Server @features_1253_p If you want to access the same database at the same time from different processes or computers, you need to use the client / server mode. In this case, one process acts as the server, and the other processes (that could reside on other computers as well) connect to the server via TCP/IP (or SSL/TLS over TCP/IP for improved security). @features_1254_h3 Multithreading Support @features_1255_p This database is multithreading-safe. That means, if an application is multi-threaded, it does not need o worry about synchronizing the access to the database. Internally, most requests to the same database are synchronized. That means an application can use multiple threads all accessing the same database at the same time, however if one thread executes a long running query, the other threads need to wait. @features_1256_h3 Locking, Lock-Timeout, Deadlocks @features_1257_p The database uses table level locks to give each connection a consistent state of the data. There are two kinds of locks: read locks (shared locks) and write locks (exclusive locks). If a connection wants to reads from a table, and there is no write lock on the table, then a read lock is added to the table. If there is a write lock, then this connection waits for the other connection to release the lock. If connection cannot get a lock for a specified time, then a lock timeout exception is thrown. @features_1258_p Usually, SELECT statement will generate read locks. This includes subqueries. Statements that modify data use write locks. It is also possible to lock a table exclusively without modifying data, using the statement SELECT ... FOR UPDATE. The statements COMMIT and ROLLBACK releases all open locks. The commands SAVEPOINT and ROLLBACK TO SAVEPOINT don't affect locks. The locks are also released when the autocommit mode changes, and for connections with autocommit set to true (this is the default), locks are released after each statement. Here is an overview on what statements generate what type of lock: @features_1259_th Type of Lock @features_1260_th SQL Statement @features_1261_td Read @features_1262_td SELECT * FROM TEST @features_1263_td CALL SELECT MAX(ID) FROM TEST @features_1264_td SCRIPT @features_1265_td Write @features_1266_td SELECT * FROM TEST WHERE 1=0 FOR UPDATE @features_1267_td Write @features_1268_td INSERT INTO TEST VALUES(1, 'Hello') @features_1269_td INSERT INTO TEST SELECT * FROM TEST @features_1270_td UPDATE TEST SET NAME='Hi' @features_1271_td DELETE FROM TEST @features_1272_td Write @features_1273_td ALTER TABLE TEST ... @features_1274_td CREATE INDEX ... ON TEST ... @features_1275_td DROP INDEX ... @features_1276_p The number of seconds until a lock timeout exception is thrown can be set separately for each connection using the SQL command SET LOCK_TIMEOUT <milliseconds>. The initial lock timeout (that is the timeout used for new connections) can be set using the SQL command SET DEFAULT_LOCK_TIMEOUT <milliseconds>. The default lock timeout is persistent. @features_1277_h2 Database File Layout @features_1278_p There are a number of files created for persistent databases. Other than some databases, not every table and/or index is stored in its own file. Instead, usually only the following files are created: A data file, an index file, a log file, and a database lock file (exists only while the database is in use). In addition to that, a file is created for each large object (CLOB/BLOB), a file for each linear index, and temporary files for large result sets. Then the command SCRIPT can create script files. If the database trace option is enabled, trace files are created. The following files can be created by the database: @features_1279_th File Name @features_1280_th Description @features_1281_th Number of Files @features_1282_td test.data.db @features_1283_td Data file @features_1284_td Contains the data for all tables @features_1285_td Format: <database>.data.db @features_1286_td 1 per database @features_1287_td test.index.db @features_1288_td Index file @features_1289_td Contains the data for all (btree) indexes @features_1290_td Format: <database>.index.db @features_1291_td 1 per database @features_1292_td test.0.log.db @features_1293_td Log file @features_1294_td The log file is used for recovery @features_1295_td Format: <database>.<id>.log.db @features_1296_td 0 or more per database @features_1297_td test.lock.db @features_1298_td Database lock file @features_1299_td Exists only if the database is open @features_1300_td Format: <database>.lock.db @features_1301_td 1 per database @features_1302_td test.trace.db @features_1303_td Trace file @features_1304_td Contains trace information @features_1305_td Format: <database>.trace.db @features_1306_td If the file is too big, it is renamed to <database>.trace.db.old @features_1307_td 1 per database @features_1308_td test.14.15.lob.db @features_1309_td Large object @features_1310_td Contains the data for BLOB or CLOB @features_1311_td Format: <database>.<tableid>.<id>.lob.db @features_1312_td 1 per object @features_1313_td test.123.temp.db @features_1314_td Temporary file @features_1315_td Contains a temporary blob or a large result set @features_1316_td Format: <database>.<session id>.<object id>.temp.db @features_1317_td 1 per object @features_1318_td test.7.hash.db @features_1319_td Hash index file @features_1320_td Contains the data for a linear hash index @features_1321_td Format: <database>.<object id>.hash.db @features_1322_td 1 per linear hash index @features_1323_h3 Moving and Renaming Database Files @features_1324_p Database name and location are not stored inside the database names. @features_1325_p While a database is closed, the files can be moved to another directory, and they can be renamed as well (as long as all files start with the same name). @features_1326_p As there is no platform specific data in the files, they can be moved to other operating systems without problems. @features_1327_h3 Backup @features_1328_p When the database is closed, it is possible to backup the database files. Please note that index files do not need to be backed up, because they contain redundant data, and will be recreated automatically if they don't exist. @features_1329_p To backup data while the database is running, the SQL command SCRIPT can be used. @features_1330_h2 Logging and Recovery @features_1331_p Whenever data is modified in the database and those changes are committed, the changes are logged to disk (except for in-memory objects). The changes to the data file itself are usually written later on, to optimize disk access. If there is a power failure, the data and index files are not up-to-date. But because the changes are in the log file, the next time the database is opened, the changes that are in the log file are re-applied automatically. @features_1332_p Please note that index file updates are not logged by default. If the database is opened and recovery is required, the index file is rebuilt from scratch. @features_1333_p There is usually only one log file per database. This file grows until the database is closed successfully, and is then deleted. Or, if the file gets too big, the database switches to another log file (with a higher id). It is possible to force the log switching by using the CHECKPOINT command. @features_1334_p If the database file is corrupted, because the checksum of a record does not match (for example, if the file was edited with another application), the database can be opened in recovery mode. In this case, errors in the database are logged but not thrown. The database should be backed up to a script and re-built as soon as possible. To open the database in the recovery mode, use a database URL must contain RECOVER=1, as in jdbc:h2:~/test;RECOVER=1. Indexes are rebuilt in this case, and the summary (object allocation table) is not read in this case, so opening the database takes longer. @features_1335_h2 Compatibility @features_1336_p All database engines behave a little bit different. Where possible, H2 supports the ANSI SQL standard, and tries to be compatible to other databases. There are still a few differences however: @features_1337_p In MySQL text columns are case insensitive by default, while in H2 they are case sensitive. However H2 supports case insensitive columns as well. To create the tables with case insensitive texts, append IGNORECASE=TRUE to the database URL (example: jdbc:h2:test;IGNORECASE=TRUE). @features_1338_h3 Compatibility Modes @features_1339_p For certain features, this database can emulate the behavior of specific databases. Not all features or differences of those databases are implemented. Currently, this feature is mainly used for randomized comparative testing (where random statements are executed against multiple databases and the results are compared). The mode can be changed by specifying the mode in the database URL, or using the SQL statement SET MODE. To use the HSQLDB mode, you can use the database URL jdbc:h2:~/test;MODE=HSQLDB or the SQL statement SET MODE HSQLDB . Here is the list of currently supported modes and the difference to the regular mode: @features_1340_th Mode @features_1341_th Differences @features_1342_td PostgreSQL @features_1343_td Concatenation of a NULL with another value results in NULL. Usually, the NULL is treated as an empty string if only one of the operators is NULL, and NULL is only returned if both values are NULL. @features_1344_td MySQL @features_1345_td When inserting data, if a column is defined to be NOT NULL and NULL is inserted, then a 0 (or empty string, or the current timestamp for timestamp columns) value is used. Usually, this operation is not allowed and an exception is thrown. @features_1346_td HSQLDB @features_1347_td When converting the scale of decimal data, the number is only converted if the new scale is smaller then current scale. Usually, the scale is converted and 0s are added if required. @features_1348_h2 Using the Trace Options @features_1349_p To find problems in an application, it is sometimes good to see what database operations where executed. This database offers the following trace features: @features_1350_li Trace to System.out and/or a file @features_1351_li Support for trace levels OFF, ERROR, INFO, and DEBUG @features_1352_li The maximum size of the trace file can be set @features_1353_li The Java code generation is possible @features_1354_li Trace can be enabled at runtime by manually creating a file @features_1355_h3 Trace Options @features_1356_p The simplest way to enable the trace option is setting it in the database URL. There are two settings, one for System.out (TRACE_LEVEL_SYSTEM_OUT) tracing, and one for file tracing (TRACE_LEVEL_FILE). The trace levels are 0 for OFF, 1 for ERROR (the default), 2 for INFO and 3 for DEBUG. A database URL with both levels set to DEBUG is: @features_1357_p The trace level can be changed at runtime by executing the SQL command SET TRACE_LEVEL_SYSTEM_OUT level (for System.out tracing) or SET TRACE_LEVEL_FILE level (for file tracing). Example: @features_1358_h3 Setting the Maximum Size of the Trace File @features_1359_p When using a high trace level, the trace file can get very big quickly. The size of the file can be limited by executing the SQL statement SET TRACE_MAX_FILE_SIZE maximumFileSizeInMB . If the log file exceeds the limit, the file is renamed to .old and a new file is created. If another .old file exists, it is deleted. The default setting is 16 MB. Example: @features_1360_h3 Java Code Generation @features_1361_p When setting the trace level to INFO or DEBUG, Java source code is generated as well, so that problem can be reproduced more easily. The trace file looks like this: @features_1362_p You need to filter out the lines without /**/ to get the Java source code. In Windows, a simple way to do that is: @features_1363_p Afterwards, you need to complete the file Trace.java before it can be compiled, for example with: @features_1364_p Also, the user name and password needs to be set, because they are not listed in the trace file. @features_1365_h3 Enabling the Trace Option at Runtime by Manually Creating a File @features_1366_p Sometimes, you can't or don't want to change the application or database URL. There is still a way to enable the trace mode in these cases, even at runtime (while the database connection is open). You only need to create a special file in the directory where the database files are stored. The database engine checks every 4 seconds if this file exists (only while executing a statement). The file name is the database name plus '.trace.db.start'. This feature is disabled if the database is encrypted. @features_1367_p Example: if a database is called 'test', then the file to start tracing is 'test.trace.db.start'. The database engine tries to delete this file when it detects it. If trace is enabled using the start file, the trace level is not persistent to the database, and trace is switched back to the level that was set before when connecting to the database. However, if the start file is read only, the database engine cannot delete the file and will always enable the trace mode when connecting. @features_1368_h2 Read Only Databases @features_1369_p If the database files are read-only, then the database is read-only as well. It is not possible to create new tables, add or modify data in this database. Only SELECT statements are allowed. To create a read-only database, close the database so that the log file gets smaller. Do not delete the log file. Then, make the database files read-only using the operating system. When you open the database now, it is read-only. There are two ways an application can find out a database is read-only: By calling Connection.isReadOnly() or by executing the SQL statement CALL READONLY(). @features_1370_h2 Read Only Databases in Zip or Jar File @features_1371_p To create a read-only database in a zip, first create a regular persistent database, and then create a backup. If you are using a database named 'test', an easy way to do that is using the BACKUP SQL statement: @features_1372_p Afterwards, you can log out, and directly open the database in the zip file using the following database URL: @features_1373_p Databases in a zip file are read-only. The performance for some queries will be slower than when using a regular database, because random access in zip files is not supported (only streaming). How much this affects the performance depends on the queries and the data. The database is not read in memory, so large databases are supported as well. The same indexes are used than when using a regular database. @features_1374_h2 Binary and Text Storage Formats @features_1375_p This database engine supports both binary and text storage formats. The binary format is faster, but the text storage format can be useful as well, for example to debug the database engine. If a database already exists, the storage format is recognized automatically. New databases are created in the binary storage format by default. To create a new database in the text storage format, the database URL must contain the parameter STORAGE=TEXT. Example URL: jdbc:h2:~/test;STORAGE=TEXT @features_1376_h2 Graceful Handling of Low Disk Space Situations @features_1377_p The database is able to deal with situations where the disk space available is running low. Whenever the database starts, an 'emergency space' file is created (size is 1 MB), and if there is no more space available, the file will shrink. If the space available is lower than 128 KB, the database will go into a special read only mode, where writing operations are no longer allowed: All writing operations will throw the exception 'No disk space available' from this point on. To go back to the normal operating mode, all connections to the database need to be closed first, and space needs to be freed up. @features_1378_p It is possible to install a database event listener to detect low disk space situations early on (when only 1 MB if space is available). To do this, use the SQL statement SET DATABASE_EVENT_LISTENER. The listener can also be set at connection time, using an URL of the form jdbc:h2:~/test;DATABASE_EVENT_LISTENER='com.acme.DbListener' (the quotes around the class name are required). See also the DatabaseEventListener API. @features_1379_h3 Opening a Corrupted Database @features_1380_p If a database can not be opened because the boot info (the SQL script that is run at startup) is corrupted, then the database can be opened by specifying a database event listener. The exceptions are logged, but opening the database will continue. @features_1381_h2 Computed Columns / Function Based Index @features_1382_p Function indexes are not directly supported by this database, but they can be easily emulated by using computed columns. For example, if an index on the upper-case version of a column is required, just create a computed column with the upper-case version of the original column, and index this column: @features_1383_p When inserting data, it is not required (better: not allowed) to specify a value for the upper-case version of the column, because the value is generated. But you can use the column when querying the table: @features_1384_h2 Multi-Dimensional Indexes @features_1385_p A tool is provided to execute efficient multi-dimension (spatial) range queries. This database does not support a specialized spatial index (R-Tree or similar). Instead, the B-Tree index is used. For each record, the multi-dimensional key is converted (mapped) to a single dimensional (scalar) value. This value specifies the location on a space-filling curve. @features_1386_p Currently, Z-order (also called N-order or Morton-order) is used; Hilbert curve could also be used, but the implementation is more complex. The algorithm to convert the multi-dimensional value is called bit-interleaving. The scalar value is indexed using a B-Tree index (usually using a computed column). @features_1387_p The method can result in a drastic performance improvement over just using an index on the first column. Depending on the data and number of dimensions, the improvement is usually higher than factor 5. The tool generates a SQL query from a specified multi-dimensional range. The method used is not database dependent, and the tool can easily be ported to other databases. For an example how to use the tool, please have a look at the sample code provided in TestMultiDimension.java. @features_1388_h2 Using Passwords @features_1389_h3 Using Secure Passwords @features_1390_p Remember that weak passwords can be broken no matter of the encryption and security protocol. Don't use passwords that can be found in a dictionary. Also appending numbers does not make them secure. A way to create good passwords that can be remembered is, take the first letters of a sentence, use upper and lower case characters, and creatively include special characters. Example: @features_1391_p i'sE2rtPiUKtT (it's easy to remember this password if you know the trick) @features_1392_h3 Passwords: Using Char Arrays instead of Strings @features_1393_p Java Strings are immutable objects and cannot be safely 'destroyed' by the application. After creating a String, it will remain in the main memory of the computer at least until it is garbage collected. The garbage collection cannot be controlled by the application, and even if it is garbage collected the data may still remain in memory. It might also be possible that the part of memory containing the password is swapped to disk (because not enough main memory is available). @features_1394_p An attacker might have access to the swap file of the operating system. It is therefore a good idea to use char arrays instead of Strings to store passwords. Char arrays can be cleared (filled with zeros) after use, and therefore the password will not be stored in the swap file. @features_1395_p This database supports using char arrays instead of String to pass user and file passwords. The following code can be used to do that: @features_1396_p In this example, the password is hard code in the application, which is not secure of course. However, Java Swing supports a way to get passwords using a char array (JPasswordField). @features_1397_h3 Passing the User Name and/or Password in the URL @features_1398_p Instead of passing the user name as a separate parameter as in Connection conn = DriverManager. getConnection("jdbc:h2:~/test", "sa", "123"); the user name (and/or password) can be supplied in the URL itself: Connection conn = DriverManager. getConnection("jdbc:h2:~/test;USER=sa;PASSWORD=123"); The settings in the URL override the settings passed as a separate parameter. @features_1399_h2 User Defined Functions and Stored Procedures @features_1400_p In addition to the built-in functions, this database supports user defined Java functions. In this database, Java functions can be used as stored procedures as well. A function must be declared (registered) before it can be used. Only static Java methods are supported; both the class and the method must be public. Example Java method: @features_1401_p The Java function must be registered in the database by calling CREATE ALIAS: @features_1402_p For a complete sample application, see src/test/org/h2/samples/Function.java. @features_1403_h3 Function Data Type Mapping @features_1404_p Functions that accept non-nullable parameters such as 'int' will not be called if one of those parameters is NULL. In this case, the value NULL is used as the result. If the function should be called in this case, you need to use 'java.lang.Integer' instead of 'int'. @features_1405_h3 Functions that require a Connection @features_1406_p If the first parameter in a Java function is a java.sql.Connection, then the connection to database is provided. This connection does not need to be closed before returning. @features_1407_h3 Functions throwing an Exception @features_1408_p If a function throws an Exception, then the current statement is rolled back and the exception is thrown to the application. @features_1409_h3 Functions returning a Result Set @features_1410_p Functions may returns a result set. Such a function can be called with the CALL statement: @features_1411_h3 Using SimpleResultSet @features_1412_p A function that returns a result set can create this result set from scratch using the SimpleResultSet tool: @features_1413_h3 Using a Function as a Table @features_1414_p A function returning a result set can be like a table. However, in this case the function is called at least twice: First while parsing the statement to collect the column names (with parameters set to null where not known at compile time). And then, while executing the statement to get the data (may be repeatedly if this is a join). If the function is called just to get the column list, the URL of the connection passed to the function is jdbc:columnlist:connection. Otherwise, the URL of the connection is jdbc:default:connection. @features_1415_h2 Triggers @features_1416_p This database supports Java triggers that are called before or after a row is updated, inserted or deleted. Triggers can be used for complex consistency checks, or to update related data in the database. It is also possible to use triggers to simulate materialized views. For a complete sample application, see src/test/org/h2/samples/TriggerSample.java. A Java trigger must implement the interface org.h2.api.Trigger: @features_1417_p The connection can be used to query or update data in other tables. The trigger then needs to be defined in the database: @features_1418_p The trigger can be used to veto a change, by throwing a SQL Exception. @features_1419_h2 Compacting a Database @features_1420_p Empty space in the database file is re-used automatically. To re-build the indexes, the most simple way is to delete the .index.db file while the database is closed. However in some situations (for example after deleting a lot of data in a database), one sometimes wants to shrink the size of the database (compact a database). Here is a sample function to do this: @features_1421_p See also the sample application org.h2.samples.Compact. The commands SCRIPT / RUNSCRIPT can be used as well to create the a backup of a database and re-build the database from the script. @features_1422_h2 Cache Settings @features_1423_p The database keeps most frequently used data and index pages in the main memory. The amount of memory used for caching can be changed using the setting CACHE_SIZE. This setting can be set in the database connection URL (jdbc:h2:~/test;CACHE_SIZE=131072), or it can be changed at runtime using SET CACHE_SIZE size. @features_1424_p This database supports two cache page replacement algorithms: LRU (the default) and 2Q. For LRU, the pages that were least frequently used are removed from the cache if it becomes full. The 2Q algorithm is a bit more complicated, basically two queues are used. The 2Q algorithm is more resistant to table scans, however the overhead is a bit higher compared to the LRU. To use the cache algorithm 2Q, use a database URL of the form jdbc:h2:~/test;CACHE_TYPE=TQ. The cache algorithm can not be changed once the database is open. @features_1425_p To get information about page reads and writes, and the current caching algorithm in use, call SELECT * FROM INFORMATION_SCHEMA.SETTINGS. The number of pages read / written is listed for the data and index file. @features_1426_h2 Why Java @features_1427_p A few reasons using a Java database are: @features_1428_li Very simple to integrate in Java applications @features_1429_li Support for many different platforms @features_1430_li More secure than native applications (no buffer overflows) @features_1431_li User defined functions (or triggers) run very fast @features_1432_li Unicode support @features_1433_p Some people think that Java is still too slow for low level operations, but this is not the case (not any more). In general, the code can be written a lot faster than using C or C++. Like that, it is possible to concentrate on improving the algorithms (that make the application faster) rather than porting the code and dealing with low level stuff (such as memory management or dealing with threads). Garbage collection is now probably faster than manual memory management. @features_1434_p A lot of features are already built in (for example Unicode, network libraries). It is very easy to write secure code because buffer overflows and such problems can be detected very easily. Some features such as the reflection mechanism can be used for randomized testing. @features_1435_p Java is also future proof: A lot of companies support Java, and it is now open source. @features_1436_p This software does not rely on many Java libraries or other software, to increase the portability and ease of use, and for performance reasons. For example, the encryption algorithms and many library functions are implemented in the database instead of using the existing libraries. Libraries that are not available in open source Java implementations (such as Swing) are not used or only used for specific features. @frame_1000_p H2 (for 'Hypersonic 2') is free a Java SQL DBMS. Clustering, embedded and server mode, transactions, referential integrity, views, subqueries, triggers, encryption, and disk based or in-memory operation are supported. A browser based console application is included. If you see this page your browser does not support frames. Please click here to view the index . @history_1000_h1 History and Roadmap @history_1001_a History of this Database Engine @history_1002_a Change Log @history_1003_a Roadmap @history_1004_a Supporters @history_1005_h2 History of this Database Engine @history_1006_p The development of H2 was started in May 2004, but it was first published on December 14th 2005. The author of H2, Thomas Mueller, is also the original developer of Hypersonic SQL. In 2001, he joined PointBase Inc. where he created PointBase Micro. At that point, he had to discontinue Hypersonic SQL, but then the HSQLDB Group was formed to continued to work on the Hypersonic SQL codebase. The name H2 stands for Hypersonic 2; however H2 does not share any code with Hypersonic SQL or HSQLDB. H2 is built from scratch. @history_1007_h2 Change Log @history_1008_p The up-to-date change log is available here: http://groups.google.com/group/h2-database/web/change-log @history_1009_h2 Roadmap @history_1010_p The current roadmap is available here: http://groups.google.com/group/h2-database/web/roadmap @history_1011_h3 Not Planned @history_1012_li HSQLDB does/did support this: select id i from test where i>0 (other databases don't) @history_1013_li String.intern (so that Strings can be compared with ==) will not be used because some VMs have problems when used extensively @history_1014_h2 Supporters @history_1015_p Many thanks for those who helped by finding and reporting bugs, gave valuable feedback, spread the word and have translated this project. Also many thanks to the donors who contributed via PayPal: @history_1016_li Florent Ramiere, France @history_1017_li Pete Haidinyak, USA @history_1018_li Jun Iyama, Japan @history_1019_li Antonio Casqueiro, Portugal @history_1020_li lumber-mill.co.jp, Japan @history_1021_li Oliver Computing LLC, USA @installation_1000_h1 Installation @installation_1001_a Requirements @installation_1002_a Supported Platforms @installation_1003_a Installing the Software @installation_1004_a Directory Structure @installation_1005_h2 Requirements @installation_1006_p To run the database, the following minimum software stack is known to work: @installation_1007_li Windows XP, MacOS, or Linux @installation_1008_li Recommended Windows file system: NTFS (FAT32 supports files up to 4 GB) @installation_1009_li Sun JDK 1.4 or newer @installation_1010_li Mozilla Firefox 1.5 or newer @installation_1011_h2 Supported Platforms @installation_1012_p As this database is written in Java, it can be run on many different platforms. It is tested with Java 1.4, 1.5, and 1.6 but can also be compiled to native code using GCJ. The source code does not use features of Java 1.5. Currently, the database is developed and tested on Windows XP using the Sun JDK 1.4, but it also works in many other operating systems and using other Java runtime environments. @installation_1013_h2 Installing the Software @installation_1014_p To install the software, run the installer or unzip it to a directory of your choice. @installation_1015_h2 Directory Structure @installation_1016_p After installing, you should get the following directory structure: @installation_1017_th Directory @installation_1018_th Contents @installation_1019_td bin @installation_1020_td JAR and batch files @installation_1021_td docs @installation_1022_td Documentation @installation_1023_td docs/html @installation_1024_td HTML pages @installation_1025_td docs/javadoc @installation_1026_td Javadoc files @installation_1027_td service @installation_1028_td Tools to run the database as a Windows Service @installation_1029_td src @installation_1030_td Source files @license_1000_h1 License @license_1001_h2 Summary and License FAQ @license_1002_p This license is a modified version of the MPL 1.1 available at www.mozilla.org/MPL , the changes are @license_1003_em underlined . There is a License FAQ section at the Mozilla web site, most of that is applicable to the H2 License as well. @license_1004_li You can use H2 for free. You can integrate it into your application (including commercial applications), and you can distribute it. @license_1005_li Files containing only your code are not covered by this license (it is 'commercial friendly'). @license_1006_li Modifications to the H2 source code must be published. @license_1007_li You don't need to provide the source code of H2 if you did not modify anything. @license_1008_p However, nobody is allowed to rename H2, modify it a little, and sell it as a database engine without telling the customers it is in fact H2. This happened to HSQLDB, when a company called 'bungisoft' copied HSQLDB, renamed it to 'RedBase', and tried to sell it, hiding the fact that it was, in fact, just HSQLDB. At this time, it seems 'bungisoft' does not exist any more, but you can use the Wayback Machine of http://www.archive.org and look for old web pages of http://www.bungisoft.com. @license_1009_p About porting the source code to another language (for example C# or C++): Converted source code (even if done manually) stays under the same copyright and license as the original code. The copyright of the ported source code does not (automatically) go to the person who ported the code. @license_1010_h2 H2 License, Version 1.0 @license_1011_h3 1. Definitions @license_1012_b 1.0.1. "Commercial Use" @license_1013_p means distribution or otherwise making the Covered Code available to a third party. @license_1014_b 1.1. "Contributor" @license_1015_p means each entity that creates or contributes to the creation of Modifications. @license_1016_b 1.2. "Contributor Version" @license_1017_p means the combination of the Original Code, prior Modifications used by a Contributor, and the Modifications made by that particular Contributor. @license_1018_b 1.3. "Covered Code" @license_1019_p means the Original Code or Modifications or the combination of the Original Code and Modifications, in each case including portions thereof. @license_1020_b 1.4. "Electronic Distribution Mechanism" @license_1021_p means a mechanism generally accepted in the software development community for the electronic transfer of data. @license_1022_b 1.5. "Executable" @license_1023_p means Covered Code in any form other than Source Code. @license_1024_b 1.6. "Initial Developer" @license_1025_p means the individual or entity identified as the Initial Developer in the Source Code notice required by Exhibit A . @license_1026_b 1.7. "Larger Work" @license_1027_p means a work which combines Covered Code or portions thereof with code not governed by the terms of this License. @license_1028_b 1.8. "License" @license_1029_p means this document. @license_1030_b 1.8.1. "Licensable" @license_1031_p means having the right to grant, to the maximum extent possible, whether at the time of the initial grant or subsequently acquired, any and all of the rights conveyed herein. @license_1032_b 1.9. "Modifications" @license_1033_p means any addition to or deletion from the substance or structure of either the Original Code or any previous Modifications. When Covered Code is released as a series of files, a Modification is: @license_1034_p 1.9.a. Any addition to or deletion from the contents of a file containing Original Code or previous Modifications. @license_1035_p 1.9.b. Any new file that contains any part of the Original Code or previous Modifications. @license_1036_b 1.10. "Original Code" @license_1037_p means Source Code of computer software code which is described in the Source Code notice required by Exhibit A as Original Code, and which, at the time of its release under this License is not already Covered Code governed by this License. @license_1038_b 1.10.1. "Patent Claims" @license_1039_p means any patent claim(s), now owned or hereafter acquired, including without limitation, method, process, and apparatus claims, in any patent Licensable by grantor. @license_1040_b 1.11. "Source Code" @license_1041_p means the preferred form of the Covered Code for making modifications to it, including all modules it contains, plus any associated interface definition files, scripts used to control compilation and installation of an Executable, or source code differential comparisons against either the Original Code or another well known, available Covered Code of the Contributor's choice. The Source Code can be in a compressed or archival form, provided the appropriate decompression or de-archiving software is widely available for no charge. @license_1042_b 1.12. "You" (or "Your") @license_1043_p means an individual or a legal entity exercising rights under, and complying with all of the terms of, this License or a future version of this License issued under Section 6.1. For legal entities, "You" includes any entity which controls, is controlled by, or is under common control with You. For purposes of this definition, "control" means (a) the power, direct or indirect, to cause the direction or management of such entity, whether by contract or otherwise, or (b) ownership of more than fifty percent (50%) of the outstanding shares or beneficial ownership of such entity. @license_1044_h3 2. Source Code License @license_1045_h4 2.1. The Initial Developer Grant @license_1046_p The Initial Developer hereby grants You a world-wide, royalty-free, non-exclusive license, subject to third party intellectual property claims: @license_1047_p 2.1.a. under intellectual property rights (other than patent or trademark) Licensable by Initial Developer to use, reproduce, modify, display, perform, sublicense and distribute the Original Code (or portions thereof) with or without Modifications, and/or as part of a Larger Work; and @license_1048_p 2.1.b. under Patents Claims infringed by the making, using or selling of Original Code, to make, have made, use, practice, sell, and offer for sale, and/or otherwise dispose of the Original Code (or portions thereof). @license_1049_p 2.1.c. the licenses granted in this Section 2.1 ( a ) and ( b ) are effective on the date Initial Developer first distributes Original Code under the terms of this License. @license_1050_p 2.1.d. Notwithstanding Section 2.1 ( b ) above, no patent license is granted: 1) for code that You delete from the Original Code; 2) separate from the Original Code; or 3) for infringements caused by: i) the modification of the Original Code or ii) the combination of the Original Code with other software or devices. @license_1051_h4 2.2. Contributor Grant @license_1052_p Subject to third party intellectual property claims, each Contributor hereby grants You a world-wide, royalty-free, non-exclusive license @license_1053_p 2.2.a. under intellectual property rights (other than patent or trademark) Licensable by Contributor, to use, reproduce, modify, display, perform, sublicense and distribute the Modifications created by such Contributor (or portions thereof) either on an unmodified basis, with other Modifications, as Covered Code and/or as part of a Larger Work; and @license_1054_p 2.2.b. under Patent Claims infringed by the making, using, or selling of Modifications made by that Contributor either alone and/or in combination with its Contributor Version (or portions of such combination), to make, use, sell, offer for sale, have made, and/or otherwise dispose of: 1) Modifications made by that Contributor (or portions thereof); and 2) the combination of Modifications made by that Contributor with its Contributor Version (or portions of such combination). @license_1055_p 2.2.c. the licenses granted in Sections 2.2 ( a ) and 2.2 ( b ) are effective on the date Contributor first makes Commercial Use of the Covered Code. @license_1056_p 2.2.c. Notwithstanding Section 2.2 ( b ) above, no patent license is granted: 1) for any code that Contributor has deleted from the Contributor Version; 2) separate from the Contributor Version; 3) for infringements caused by: i) third party modifications of Contributor Version or ii) the combination of Modifications made by that Contributor with other software (except as part of the Contributor Version) or other devices; or 4) under Patent Claims infringed by Covered Code in the absence of Modifications made by that Contributor. @license_1057_h3 3. Distribution Obligations @license_1058_h4 3.1. Application of License @license_1059_p The Modifications which You create or to which You contribute are governed by the terms of this License, including without limitation Section 2.2 . The Source Code version of Covered Code may be distributed only under the terms of this License or a future version of this License released under Section 6.1 , and You must include a copy of this License with every copy of the Source Code You distribute. You may not offer or impose any terms on any Source Code version that alters or restricts the applicable version of this License or the recipients' rights hereunder. However, You may include an additional document offering the additional rights described in Section 3.5 . @license_1060_h4 3.2. Availability of Source Code @license_1061_p Any Modification which You create or to which You contribute must be made available in Source Code form under the terms of this License either on the same media as an Executable version or via an accepted Electronic Distribution Mechanism to anyone to whom you made an Executable version available; and if made available via Electronic Distribution Mechanism, must remain available for at least twelve (12) months after the date it initially became available, or at least six (6) months after a subsequent version of that particular Modification has been made available to such recipients. You are responsible for ensuring that the Source Code version remains available even if the Electronic Distribution Mechanism is maintained by a third party. @license_1062_h4 3.3. Description of Modifications @license_1063_p You must cause all Covered Code to which You contribute to contain a file documenting the changes You made to create that Covered Code and the date of any change. You must include a prominent statement that the Modification is derived, directly or indirectly, from Original Code provided by the Initial Developer and including the name of the Initial Developer in (a) the Source Code, and (b) in any notice in an Executable version or related documentation in which You describe the origin or ownership of the Covered Code. @license_1064_h4 3.4. Intellectual Property Matters @license_1065_b 3.4.a. Third Party Claims: @license_1066_p If Contributor has knowledge that a license under a third party's intellectual property rights is required to exercise the rights granted by such Contributor under Sections 2.1 or 2.2 , Contributor must include a text file with the Source Code distribution titled "LEGAL" which describes the claim and the party making the claim in sufficient detail that a recipient will know whom to contact. If Contributor obtains such knowledge after the Modification is made available as described in Section 3.2 , Contributor shall promptly modify the LEGAL file in all copies Contributor makes available thereafter and shall take other steps (such as notifying appropriate mailing lists or newsgroups) reasonably calculated to inform those who received the Covered Code that new knowledge has been obtained. @license_1067_b 3.4.b. Contributor APIs: @license_1068_p If Contributor's Modifications include an application programming interface and Contributor has knowledge of patent licenses which are reasonably necessary to implement that API, Contributor must also include this information in the legal file. @license_1069_b 3.4.c. Representations: @license_1070_p Contributor represents that, except as disclosed pursuant to Section 3.4 ( a ) above, Contributor believes that Contributor's Modifications are Contributor's original creation(s) and/or Contributor has sufficient rights to grant the rights conveyed by this License. @license_1071_h4 3.5. Required Notices @license_1072_p You must duplicate the notice in Exhibit A in each file of the Source Code. If it is not possible to put such notice in a particular Source Code file due to its structure, then You must include such notice in a location (such as a relevant directory) where a user would be likely to look for such a notice. If You created one or more Modification(s) You may add your name as a Contributor to the notice described in Exhibit A . You must also duplicate this License in any documentation for the Source Code where You describe recipients' rights or ownership rights relating to Covered Code. You may choose to offer, and to charge a fee for, warranty, support, indemnity or liability obligations to one or more recipients of Covered Code. However, You may do so only on Your own behalf, and not on behalf of the Initial Developer or any Contributor. You must make it absolutely clear than any such warranty, support, indemnity or liability obligation is offered by You alone, and You hereby agree to indemnify the Initial Developer and every Contributor for any liability incurred by the Initial Developer or such Contributor as a result of warranty, support, indemnity or liability terms You offer. @license_1073_h4 3.6. Distribution of Executable Versions @license_1074_p You may distribute Covered Code in Executable form only if the requirements of Sections 3.1 , 3.2 , 3.3 , 3.4 and 3.5 have been met for that Covered Code, and if You include a notice stating that the Source Code version of the Covered Code is available under the terms of this License, including a description of how and where You have fulfilled the obligations of Section 3.2 . The notice must be conspicuously included in any notice in an Executable version, related documentation or collateral in which You describe recipients' rights relating to the Covered Code. You may distribute the Executable version of Covered Code or ownership rights under a license of Your choice, which may contain terms different from this License, provided that You are in compliance with the terms of this License and that the license for the Executable version does not attempt to limit or alter the recipient's rights in the Source Code version from the rights set forth in this License. If You distribute the Executable version under a different license You must make it absolutely clear that any terms which differ from this License are offered by You alone, not by the Initial Developer or any Contributor. You hereby agree to indemnify the Initial Developer and every Contributor for any liability incurred by the Initial Developer or such Contributor as a result of any such terms You offer. @license_1075_h4 3.7. Larger Works @license_1076_p You may create a Larger Work by combining Covered Code with other code not governed by the terms of this License and distribute the Larger Work as a single product. In such a case, You must make sure the requirements of this License are fulfilled for the Covered Code. @license_1077_h3 4. Inability to Comply Due to Statute or Regulation. @license_1078_p If it is impossible for You to comply with any of the terms of this License with respect to some or all of the Covered Code due to statute, judicial order, or regulation then You must: (a) comply with the terms of this License to the maximum extent possible; and (b) describe the limitations and the code they affect. Such description must be included in the legal file described in Section 3.4 and must be included with all distributions of the Source Code. Except to the extent prohibited by statute or regulation, such description must be sufficiently detailed for a recipient of ordinary skill to be able to understand it. @license_1079_h3 5. Application of this License. @license_1080_p This License applies to code to which the Initial Developer has attached the notice in Exhibit A and to related Covered Code. @license_1081_h3 6. Versions of the License. @license_1082_h4 6.1. New Versions @license_1083_p The @license_1084_em H2 Group may publish revised and/or new versions of the License from time to time. Each version will be given a distinguishing version number. @license_1085_h4 6.2. Effect of New Versions @license_1086_p Once Covered Code has been published under a particular version of the License, You may always continue to use it under the terms of that version. You may also choose to use such Covered Code under the terms of any subsequent version of the License published by the @license_1087_em H2 Group . No one other than the @license_1088_em H2 Group has the right to modify the terms applicable to Covered Code created under this License. @license_1089_h4 6.3. Derivative Works @license_1090_p If You create or use a modified version of this License (which you may only do in order to apply it to code which is not already Covered Code governed by this License), You must (a) rename Your license so that the phrases @license_1091_em "H2 Group", "H2" or any confusingly similar phrase do not appear in your license (except to note that your license differs from this License) and (b) otherwise make it clear that Your version of the license contains terms which differ from the @license_1092_em H2 License . (Filling in the name of the Initial Developer, Original Code or Contributor in the notice described in Exhibit A shall not of themselves be deemed to be modifications of this License.) @license_1093_h3 7. Disclaimer of Warranty @license_1094_p Covered code is provided under this license on an "as is" basis, without warranty of any kind, either expressed or implied, including, without limitation, warranties that the covered code is free of defects, merchantable, fit for a particular purpose or non-infringing. The entire risk as to the quality and performance of the covered code is with you. Should any covered code prove defective in any respect, you (not the initial developer or any other contributor) assume the cost of any necessary servicing, repair or correction. This disclaimer of warranty constitutes an essential part of this license. No use of any covered code is authorized hereunder except under this disclaimer. @license_1095_h3 8. Termination @license_1096_p 8.1. This License and the rights granted hereunder will terminate automatically if You fail to comply with terms herein and fail to cure such breach within 30 days of becoming aware of the breach. All sublicenses to the Covered Code which are properly granted shall survive any termination of this License. Provisions which, by their nature, must remain in effect beyond the termination of this License shall survive. @license_1097_p 8.2. If You initiate litigation by asserting a patent infringement claim (excluding declaratory judgment actions) against Initial Developer or a Contributor (the Initial Developer or Contributor against whom You file such action is referred to as "Participant") alleging that: @license_1098_p 8.2.a. such Participant's Contributor Version directly or indirectly infringes any patent, then any and all rights granted by such Participant to You under Sections 2.1 and/or 2.2 of this License shall, upon 60 days notice from Participant terminate prospectively, unless if within 60 days after receipt of notice You either: (i) agree in writing to pay Participant a mutually agreeable reasonable royalty for Your past and future use of Modifications made by such Participant, or (ii) withdraw Your litigation claim with respect to the Contributor Version against such Participant. If within 60 days of notice, a reasonable royalty and payment arrangement are not mutually agreed upon in writing by the parties or the litigation claim is not withdrawn, the rights granted by Participant to You under Sections 2.1 and/or 2.2 automatically terminate at the expiration of the 60 day notice period specified above. @license_1099_p 8.2.b. any software, hardware, or device, other than such Participant's Contributor Version, directly or indirectly infringes any patent, then any rights granted to You by such Participant under Sections 2.1( b ) and 2.2( b ) are revoked effective as of the date You first made, used, sold, distributed, or had made, Modifications made by that Participant. @license_1100_p 8.3. If You assert a patent infringement claim against Participant alleging that such Participant's Contributor Version directly or indirectly infringes any patent where such claim is resolved (such as by license or settlement) prior to the initiation of patent infringement litigation, then the reasonable value of the licenses granted by such Participant under Sections 2.1 or 2.2 shall be taken into account in determining the amount or value of any payment or license. @license_1101_p 8.4. In the event of termination under Sections 8.1 or 8.2 above, all end user license agreements (excluding distributors and resellers) which have been validly granted by You or any distributor hereunder prior to termination shall survive termination. @license_1102_h3 9. Limitation of Liability @license_1103_p Under no circumstances and under no legal theory, whether tort (including negligence), contract, or otherwise, shall you, the initial developer, any other contributor, or any distributor of covered code, or any supplier of any of such parties, be liable to any person for any indirect, special, incidental, or consequential damages of any character including, without limitation, damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses, even if such party shall have been informed of the possibility of such damages. This limitation of liability shall not apply to liability for death or personal injury resulting from such party's negligence to the extent applicable law prohibits such limitation. Some jurisdictions do not allow the exclusion or limitation of incidental or consequential damages, so this exclusion and limitation may not apply to you. @license_1104_h3 10. United States Government End Users @license_1105_p The Covered Code is a "commercial item", as that term is defined in 48 C.F.R. 2.101 (October 1995), consisting of "commercial computer software" and "commercial computer software documentation", as such terms are used in 48 C.F.R. 12.212 (September 1995). Consistent with 48 C.F.R. 12.212 and 48 C.F.R. 227.7202-1 through 227.7202-4 (June 1995), all U.S. Government End Users acquire Covered Code with only those rights set forth herein. @license_1106_h3 11. Miscellaneous @license_1107_p This License represents the complete agreement concerning subject matter hereof. If any provision of this License is held to be unenforceable, such provision shall be reformed only to the extent necessary to make it enforceable. This License shall be governed by @license_1108_em Swiss law provisions (except to the extent applicable law, if any, provides otherwise), excluding its conflict-of-law provisions. With respect to disputes in which at least one party is a citizen of, or an entity chartered or registered to do business in @license_1109_em Switzerland , any litigation relating to this License shall be subject to the jurisdiction of @license_1110_em Switzerland , with the losing party responsible for costs, including without limitation, court costs and reasonable attorneys' fees and expenses. The application of the United Nations Convention on Contracts for the International Sale of Goods is expressly excluded. Any law or regulation which provides that the language of a contract shall be construed against the drafter shall not apply to this License. @license_1111_h3 12. Responsibility for Claims @license_1112_p As between Initial Developer and the Contributors, each party is responsible for claims and damages arising, directly or indirectly, out of its utilization of rights under this License and You agree to work with Initial Developer and Contributors to distribute such responsibility on an equitable basis. Nothing herein is intended or shall be deemed to constitute any admission of liability. @license_1113_h3 13. Multiple-Licensed Code @license_1114_p Initial Developer may designate portions of the Covered Code as "Multiple-Licensed". "Multiple-Licensed" means that the Initial Developer permits you to utilize portions of the Covered Code under Your choice of this or the alternative licenses, if any, specified by the Initial Developer in the file described in Exhibit A . @license_1115_h3 Exhibit A @mainWeb_1000_h1 H2 Database Engine @mainWeb_1001_p Welcome to H2, the free SQL database. The main feature of H2 are: @mainWeb_1002_li Very fast, free for everybody, source code is included @mainWeb_1003_li Written Java; can be compiled with GCJ (Linux) @mainWeb_1004_li Embedded, Server and Cluster modes @mainWeb_1005_li JDBC and (partial) ODBC API; Web Client application @mainWeb_1006_h3 Download @mainWeb_1007_td Version 1.0.63 (2007-12-02): @mainWeb_1008_a Windows Installer (2.8 MB) @mainWeb_1009_a All platforms (zip, 3.9 MB) @mainWeb_1010_a All Downloads @mainWeb_1011_td     @mainWeb_1012_h3 Support @mainWeb_1013_a English Google Group @mainWeb_1014_a Japanese Google Group @mainWeb_1015_p Or send an e-mail to: @mainWeb_1016_td   @mainWeb_1017_h3 Performance @mainWeb_1018_td Operations/second (higher is better) - More information about this test @mainWeb_1019_td   @mainWeb_1020_h3 News @mainWeb_1021_b Newsfeeds: @mainWeb_1022_p Two are available: Full text (Atom) and Header only (RSS) . @mainWeb_1023_b Email Newsletter: @mainWeb_1024_p Subscribe to H2 Database News (Google account required) to get informed about new releases. Your email address is only used in this context. @mainWeb_1025_td   @mainWeb_1026_h3 Contribute @mainWeb_1027_p You can contribute to the development of H2 by sending feedback and bug reports, or translate the H2 Console application (files h2/src/main/org/h2/server/web/res/_text_*.properties). Or click on the PayPal button below to donate money. You will be listed as a supporter: @mainWeb_1028_td   @mainWeb_1029_h3 Feedback @mainWeb_1030_td You may also send questions, feature requests, or feedback of any kind here: @mainWeb_1031_p Email (optional): @mainWeb_1032_form Message: @main_1000_h1 H2 Database Engine @main_1001_p Welcome to H2, the free SQL database engine. @main_1002_a Quickstart @main_1003_p Click here to get a fast overview. @main_1004_a Tutorial @main_1005_p Go through the samples. @main_1006_a Features @main_1007_p See what this database can do and how to use these features. @performance_1000_h1 Performance @performance_1001_a Performance Comparison @performance_1002_a Application Profiling @performance_1003_a Performance Tuning @performance_1004_h2 Performance Comparison @performance_1005_p In most cases H2 is a lot faster than all other (open source and not open source) database engines. Please note this is mostly a single connection benchmark run on one computer. @performance_1006_h3 Embedded @performance_1007_th Test Case @performance_1008_th Unit @performance_1009_th H2 @performance_1010_th HSQLDB @performance_1011_th Derby @performance_1012_td Simple: Init @performance_1013_td ms @performance_1014_td 375 @performance_1015_td 578 @performance_1016_td 2797 @performance_1017_td Simple: Query (random) @performance_1018_td ms @performance_1019_td 250 @performance_1020_td 344 @performance_1021_td 1563 @performance_1022_td Simple: Query (sequential) @performance_1023_td ms @performance_1024_td 171 @performance_1025_td 250 @performance_1026_td 1469 @performance_1027_td Simple: Update (random) @performance_1028_td ms @performance_1029_td 641 @performance_1030_td 1609 @performance_1031_td 19265 @performance_1032_td Simple: Delete (sequential) @performance_1033_td ms @performance_1034_td 172 @performance_1035_td 516 @performance_1036_td 6797 @performance_1037_td Simple: Memory Usage @performance_1038_td MB @performance_1039_td 14 @performance_1040_td 12 @performance_1041_td 12 @performance_1042_td BenchA: Init @performance_1043_td ms @performance_1044_td 391 @performance_1045_td 500 @performance_1046_td 3750 @performance_1047_td BenchA: Transactions @performance_1048_td ms @performance_1049_td 5468 @performance_1050_td 2468 @performance_1051_td 16250 @performance_1052_td BenchA: Memory Usage @performance_1053_td MB @performance_1054_td 14 @performance_1055_td 15 @performance_1056_td 9 @performance_1057_td BenchB: Init @performance_1058_td ms @performance_1059_td 1281 @performance_1060_td 2391 @performance_1061_td 14938 @performance_1062_td BenchB: Transactions @performance_1063_td ms @performance_1064_td 2094 @performance_1065_td 1140 @performance_1066_td 3828 @performance_1067_td BenchB: Memory Usage @performance_1068_td MB @performance_1069_td 16 @performance_1070_td 11 @performance_1071_td 9 @performance_1072_td BenchC: Init @performance_1073_td ms @performance_1074_td 984 @performance_1075_td 547 @performance_1076_td 5250 @performance_1077_td BenchC: Transactions @performance_1078_td ms @performance_1079_td 2860 @performance_1080_td 58219 @performance_1081_td 11204 @performance_1082_td BenchC: Memory Usage @performance_1083_td MB @performance_1084_td 19 @performance_1085_td 19 @performance_1086_td 9 @performance_1087_td Executed Statements @performance_1088_td # @performance_1089_td 594255 @performance_1090_td 594255 @performance_1091_td 594255 @performance_1092_td Total Time @performance_1093_td ms @performance_1094_td 14687 @performance_1095_td 68562 @performance_1096_td 87111 @performance_1097_td Statement per Second @performance_1098_td # @performance_1099_td 40461 @performance_1100_td 8667 @performance_1101_td 6821 @performance_1102_h3 Client-Server @performance_1103_th Test Case @performance_1104_th Unit @performance_1105_th H2 @performance_1106_th HSQLDB @performance_1107_th Derby @performance_1108_th PostgreSQL @performance_1109_th MySQL @performance_1110_td Simple: Init @performance_1111_td ms @performance_1112_td 3047 @performance_1113_td 2547 @performance_1114_td 6907 @performance_1115_td 4234 @performance_1116_td 3594 @performance_1117_td Simple: Query (random) @performance_1118_td ms @performance_1119_td 3547 @performance_1120_td 2641 @performance_1121_td 8781 @performance_1122_td 5375 @performance_1123_td 3140 @performance_1124_td Simple: Query (sequential) @performance_1125_td ms @performance_1126_td 3390 @performance_1127_td 2531 @performance_1128_td 8859 @performance_1129_td 4906 @performance_1130_td 3016 @performance_1131_td Simple: Update (random) @performance_1132_td ms @performance_1133_td 3235 @performance_1134_td 3531 @performance_1135_td 22344 @performance_1136_td 5828 @performance_1137_td 5187 @performance_1138_td Simple: Delete (sequential) @performance_1139_td ms @performance_1140_td 1421 @performance_1141_td 1235 @performance_1142_td 8219 @performance_1143_td 2484 @performance_1144_td 1829 @performance_1145_td Simple: Memory Usage @performance_1146_td MB @performance_1147_td 15 @performance_1148_td 10 @performance_1149_td 15 @performance_1150_td 0 @performance_1151_td 0 @performance_1152_td BenchA: Init @performance_1153_td ms @performance_1154_td 2687 @performance_1155_td 2343 @performance_1156_td 6000 @performance_1157_td 4000 @performance_1158_td 4000 @performance_1159_td BenchA: Transactions @performance_1160_td ms @performance_1161_td 12938 @performance_1162_td 9579 @performance_1163_td 26610 @performance_1164_td 16250 @performance_1165_td 10782 @performance_1166_td BenchA: Memory Usage @performance_1167_td MB @performance_1168_td 15 @performance_1169_td 16 @performance_1170_td 10 @performance_1171_td 0 @performance_1172_td 0 @performance_1173_td BenchB: Init @performance_1174_td ms @performance_1175_td 9641 @performance_1176_td 10094 @performance_1177_td 28282 @performance_1178_td 17468 @performance_1179_td 11344 @performance_1180_td BenchB: Transactions @performance_1181_td ms @performance_1182_td 3984 @performance_1183_td 3312 @performance_1184_td 6671 @performance_1185_td 7797 @performance_1186_td 3375 @performance_1187_td BenchB: Memory Usage @performance_1188_td MB @performance_1189_td 16 @performance_1190_td 13 @performance_1191_td 8 @performance_1192_td 0 @performance_1193_td 0 @performance_1194_td BenchC: Init @performance_1195_td ms @performance_1196_td 2031 @performance_1197_td 1516 @performance_1198_td 7391 @performance_1199_td 2297 @performance_1200_td 3406 @performance_1201_td BenchC: Transactions @performance_1202_td ms @performance_1203_td 9750 @performance_1204_td 58734 @performance_1205_td 20937 @performance_1206_td 11172 @performance_1207_td 7469 @performance_1208_td BenchC: Memory Usage @performance_1209_td MB @performance_1210_td 20 @performance_1211_td 15 @performance_1212_td 14 @performance_1213_td 0 @performance_1214_td 0 @performance_1215_td Executed Statements @performance_1216_td # @performance_1217_td 594255 @performance_1218_td 594255 @performance_1219_td 594255 @performance_1220_td 594255 @performance_1221_td 594255 @performance_1222_td Total Time @performance_1223_td ms @performance_1224_td 55671 @performance_1225_td 98063 @performance_1226_td 151001 @performance_1227_td 81811 @performance_1228_td 57142 @performance_1229_td Statement per Second @performance_1230_td # @performance_1231_td 10674 @performance_1232_td 6059 @performance_1233_td 3935 @performance_1234_td 7263 @performance_1235_td 10399 @performance_1236_h3 Benchmark Results and Comments @performance_1237_h4 H2 @performance_1238_p Version 1.0 (2007-09-15) was used for the test. For simpler operations, the performance of H2 is about the same as for HSQLDB. For more complex queries, the query optimizer is very important. However H2 is not very fast in every case, certain kind of queries may still be slow. One situation where is H2 is slow is large result sets, because they are buffered to disk if more than a certain number of records are returned. The advantage of buffering is, there is no limit on the result set size. The open/close time is almost fixed, because of the file locking protocol: The engine waits 20 ms after opening a database to ensure the database files are not opened by another process. @performance_1239_h4 HSQLDB @performance_1240_p Version 1.8.0.8 was used for the test. Cached tables are used in this test (hsqldb.default_table_type=cached), and the write delay is 1 second (SET WRITE_DELAY 1). HSQLDB is fast when using simple operations. HSQLDB is very slow in the last test (BenchC: Transactions), probably because is has a bad query optimizer. One query where HSQLDB is slow is a two-table join: @performance_1241_p The PolePosition benchmark also shows that the query optimizer does not do a very good job for some queries. A disadvantage in HSQLDB is the slow startup / shutdown time (currently not listed) when using bigger databases. The reason is, a backup of the database is created whenever the database is opened or closed. @performance_1242_h4 Derby @performance_1243_p Version 10.3.1.4 was used for the test. Derby is clearly the slowest embedded database in this test. This seems to be a structural problem, because all operations are really slow. It will not be easy for the developers of Derby to improve the performance to a reasonable level. A few problems have been identified: Leaving autocommit on is a problem for Derby. If it is switched off during the whole test, the results are about 20% better for Derby. @performance_1244_h4 PostgreSQL @performance_1245_p Version 8.1.4 was used for the test. The following options where changed in postgresql.conf: fsync = off, commit_delay = 1000. PostgreSQL is run in server mode. It looks like the base performance is slower than MySQL, the reason could be the network layer. The memory usage number is incorrect, because only the memory usage of the JDBC driver is measured. @performance_1246_h4 MySQL @performance_1247_p Version 5.0.22 was used for the test. MySQL was run with the InnoDB backend. The setting innodb_flush_log_at_trx_commit (found in the my.ini file) was set to 0. Otherwise (and by default), MySQL is really slow (around 140 statements per second in this test) because it tries to flush the data to disk for each commit. For small transactions (when autocommit is on) this is really slow. But many use cases use small or relatively small transactions. Too bad this setting is not listed in the configuration wizard, and it always overwritten when using the wizard. You need to change this setting manually in the file my.ini, and then restart the service. The memory usage number is incorrect, because only the memory usage of the JDBC driver is measured. @performance_1248_h4 Firebird @performance_1249_p Firebird 1.5 (default installation) was tested, but the results are not published currently. It is possible to run the performance test with the Firebird database, and any information on how to configure Firebird for higher performance are welcome. @performance_1250_h4 Why Oracle / MS SQL Server / DB2 are Not Listed @performance_1251_p The license of these databases does not allow to publish benchmark results. This doesn't mean that they are fast. They are in fact quite slow, and need a lot of memory. But you will need to test this yourself. SQLite was not tested because the JDBC driver doesn't support transactions. @performance_1252_h3 About this Benchmark @performance_1253_h4 Number of Connections @performance_1254_p This is mostly a single-connection benchmark. BenchB uses multiple connections, the other tests one connection. @performance_1255_h4 Real-World Tests @performance_1256_p Good benchmarks emulate real-world use cases. This benchmark includes 3 test cases: A simple test case with one table and many small updates / deletes. BenchA is similar to the TPC-A test, but single connection / single threaded (see also: www.tpc.org). BenchB is similar to the TPC-B test, using multiple connections (one thread per connection). BenchC is similar to the TPC-C test, but single connection / single threaded. @performance_1257_h4 Comparing Embedded with Server Databases @performance_1258_p This is mainly a benchmark for embedded databases (where the application runs in the same virtual machine than the database engine). However MySQL and PostgreSQL are not Java databases and cannot be embedded into a Java application. For the Java databases, both embedded and server modes are tested. @performance_1259_h4 Test Platform @performance_1260_p This test is run on Windows XP with the virus scanner switched off. The VM used is Sun JDK 1.5. @performance_1261_h4 Multiple Runs @performance_1262_p When a Java benchmark is run first, the code is not fully compiled and therefore runs slower than when running multiple times. A benchmark should always run the same test multiple times and ignore the first run(s). This benchmark runs three times, the last run counts. @performance_1263_h4 Memory Usage @performance_1264_p It is not enough to measure the time taken, the memory usage is important as well. Performance can be improved in databases by using a bigger in-memory cache, but there is only a limited amount of memory available on the system. HSQLDB tables are kept fully in memory by default, this benchmark uses 'disk based' tables for all databases. Unfortunately, it is not so easy to calculate the memory usage of PostgreSQL and MySQL, because they run in a different process than the test. This benchmark currently does not print memory usage of those databases. @performance_1265_h4 Delayed Operations @performance_1266_p Some databases delay some operations (for example flushing the buffers) until after the benchmark is run. This benchmark waits between each database tested, and each database runs in a different process (sequentially). @performance_1267_h4 Transaction Commit / Durability @performance_1268_p Durability means transaction committed to the database will not be lost. Some databases (for example MySQL) try to enforce this by default by calling fsync() to flush the buffers, but most hard drives don't actually flush all data. Calling fsync() slows down transaction commit a lot, but doesn't always make data durable. When comparing the results, it is important to think about the effect. Many database suggest to 'batch' operations when possible. This benchmark switches off autocommit when loading the data, and calls commit after each 1000 inserts. However many applications need 'short' transactions at runtime (a commit after each update). This benchmark commits after each update / delete in the simple benchmark, and after each business transaction in the other benchmarks. For databases that support delayed commits, a delay of one second is used. @performance_1269_h4 Using Prepared Statements @performance_1270_p Wherever possible, the test cases use prepared statements. @performance_1271_h4 Currently Not Tested: Startup Time @performance_1272_p The startup time of a database engine is important as well for embedded use. This time is not measured currently. Also, not tested is the time used to create a database and open an existing database. Here, one (wrapper) connection is opened at the start, and for each step a new connection is opened and then closed. That means the Open/Close time listed is for opening a connection if the database is already in use. @performance_1273_h3 PolePosition Benchmark @performance_1274_p The PolePosition is an open source benchmark. The algorithms are all quite simple. It was developed / sponsored by db4o. @performance_1275_th Test Case @performance_1276_th Unit @performance_1277_th H2 @performance_1278_th HSQLDB @performance_1279_th MySQL @performance_1280_td Melbourne write @performance_1281_td ms @performance_1282_td 369 @performance_1283_td 249 @performance_1284_td 2022 @performance_1285_td Melbourne read @performance_1286_td ms @performance_1287_td 47 @performance_1288_td 49 @performance_1289_td 93 @performance_1290_td Melbourne read_hot @performance_1291_td ms @performance_1292_td 24 @performance_1293_td 43 @performance_1294_td 95 @performance_1295_td Melbourne delete @performance_1296_td ms @performance_1297_td 147 @performance_1298_td 133 @performance_1299_td 176 @performance_1300_td Sepang write @performance_1301_td ms @performance_1302_td 965 @performance_1303_td 1201 @performance_1304_td 3213 @performance_1305_td Sepang read @performance_1306_td ms @performance_1307_td 765 @performance_1308_td 948 @performance_1309_td 3455 @performance_1310_td Sepang read_hot @performance_1311_td ms @performance_1312_td 789 @performance_1313_td 859 @performance_1314_td 3563 @performance_1315_td Sepang delete @performance_1316_td ms @performance_1317_td 1384 @performance_1318_td 1596 @performance_1319_td 6214 @performance_1320_td Bahrain write @performance_1321_td ms @performance_1322_td 1186 @performance_1323_td 1387 @performance_1324_td 6904 @performance_1325_td Bahrain query_indexed_string @performance_1326_td ms @performance_1327_td 336 @performance_1328_td 170 @performance_1329_td 693 @performance_1330_td Bahrain query_string @performance_1331_td ms @performance_1332_td 18064 @performance_1333_td 39703 @performance_1334_td 41243 @performance_1335_td Bahrain query_indexed_int @performance_1336_td ms @performance_1337_td 104 @performance_1338_td 134 @performance_1339_td 678 @performance_1340_td Bahrain update @performance_1341_td ms @performance_1342_td 191 @performance_1343_td 87 @performance_1344_td 159 @performance_1345_td Bahrain delete @performance_1346_td ms @performance_1347_td 1215 @performance_1348_td 729 @performance_1349_td 6812 @performance_1350_td Imola retrieve @performance_1351_td ms @performance_1352_td 198 @performance_1353_td 194 @performance_1354_td 4036 @performance_1355_td Barcelona write @performance_1356_td ms @performance_1357_td 413 @performance_1358_td 832 @performance_1359_td 3191 @performance_1360_td Barcelona read @performance_1361_td ms @performance_1362_td 119 @performance_1363_td 160 @performance_1364_td 1177 @performance_1365_td Barcelona query @performance_1366_td ms @performance_1367_td 20 @performance_1368_td 5169 @performance_1369_td 101 @performance_1370_td Barcelona delete @performance_1371_td ms @performance_1372_td 388 @performance_1373_td 319 @performance_1374_td 3287 @performance_1375_td Total @performance_1376_td ms @performance_1377_td 26724 @performance_1378_td 53962 @performance_1379_td 87112 @performance_1380_h2 Application Profiling @performance_1381_h3 Analyze First @performance_1382_p Before trying to optimize the performance, it is important to know where the time is actually spent. The same is true for memory problems. Premature or 'blind' optimization should be avoided, as it is not an efficient way to solve the problem. There are various ways to analyze the application. In some situations it is possible to compare two implementations and use System.currentTimeMillis() to find out which one is faster. But this does not work for complex applications with many modules, and for memory problems. A very good tool to measure both the memory and the CPU is the YourKit Java Profiler . This tool is also used to optimize the performance and memory footprint of this database engine. @performance_1383_h2 Database Performance Tuning @performance_1384_h3 Virus Scanners @performance_1385_p Some virus scanners scan files every time they are accessed. It is very important for performance that database files are not scanned for viruses. The database engine does never interprets the data stored in the files as programs, that means even if somebody would store a virus in a database file, this would be harmless (when the virus does not run, it cannot spread). Some virus scanners allow excluding file endings. Make sure files ending with .db are not scanned. @performance_1386_h3 Using the Trace Options @performance_1387_p If the main performance hot spots are in the database engine, in many cases the performance can be optimized by creating additional indexes, or changing the schema. Sometimes the application does not directly generate the SQL statements, for example if an O/R mapping tool is used. To view the SQL statements and JDBC API calls, you can use the trace options. For more information, see Using the Trace Options . @performance_1388_h3 Index Usage @performance_1389_p This database uses indexes to improve the performance of SELECT, UPDATE and DELETE statements. If a column is used in the WHERE clause of a query, and if an index exists on this column, then the index can be used. Multi-column indexes are used if all or the first columns of the index are used. Both equality lookup and range scans are supported. Indexes are not used to order result sets: The results are sorted in memory if required. Indexes are created automatically for primary key and unique constraints. Indexes are also created for foreign key constraints, if required. For other columns, indexes need to be created manually using the CREATE INDEX statement. @performance_1390_h3 Optimizer @performance_1391_p This database uses a cost based optimizer. For simple and queries and queries with medium complexity (less than 7 tables in the join), the expected cost (running time) of all possible plans is calculated, and the plan with the lowest cost is used. For more complex queries, the algorithm first tries all possible combinations for the first few tables, and the remaining tables added using a greedy algorithm (this works well for most joins). Afterwards a genetic algorithm is used to test at most 2000 distinct plans. Only left-deep plans are evaluated. @performance_1392_h3 Expression Optimization @performance_1393_p After the statement is parsed, all expressions are simplified automatically if possible. Operations are evaluated only once if all parameters are constant. Functions are also optimized, but only if the function is constant (always returns the same result for the same parameter values). If the WHERE clause is always false, then the table is not accessed at all. @performance_1394_h3 COUNT(*) Optimization @performance_1395_p If the query only counts all rows of a table, then the data is not accessed. However, this is only possible if no WHERE clause is used, that means it only works for queries of the form SELECT COUNT(*) FROM table. @performance_1396_h3 Updating Optimizer Statistics / Column Selectivity @performance_1397_p When executing a query, at most one index per joined table can be used. If the same table is joined multiple times, for each join only one index is used. Example: for the query SELECT * FROM TEST T1, TEST T2 WHERE T1.NAME='A' AND T2.ID=T1.ID, two index can be used, in this case the index on NAME for T1 and the index on ID for T2. @performance_1398_p If a table has multiple indexes, sometimes more than one index could be used. Example: if there is a table TEST(ID, NAME, FIRSTNAME) and an index on each column, then two indexes could be used for the query SELECT * FROM TEST WHERE NAME='A' AND FIRSTNAME='B', the index on NAME or the index on FIRSTNAME. It is not possible to use both indexes at the same time. Which index is used depends on the selectivity of the column. The selectivity describes the 'uniqueness' of values in a column. A selectivity of 100 means each value appears only once, and a selectivity of 1 means the same value appears in many or most rows. For the query above, the index on NAME should be used if the table contains more distinct names than first names. @performance_1399_p The SQL statement ANALYZE can be used to automatically estimate the selectivity of the columns in the tables. This command should be run from time to time to improve the query plans generated by the optimizer. @quickstartText_1000_h1 Quickstart @quickstartText_1001_a Embedding H2 in an Application @quickstartText_1002_a The H2 Console Application @quickstartText_1003_h2 Embedding H2 in an Application @quickstartText_1004_p This database can be used in embedded mode, or in server mode. To use it in embedded mode, you need to: @quickstartText_1005_li Add h2.jar to the classpath @quickstartText_1006_li Use the JDBC driver class: org.h2.Driver @quickstartText_1007_li The database URL jdbc:h2:~/test opens the database 'test' in your user home directory @quickstartText_1008_h2 The H2 Console Application @quickstartText_1009_p The Console lets you access a SQL database using a browser interface. @quickstartText_1010_p If you don't have Windows XP, or if something does not work as expected, please see the detailed description in the Tutorial . @quickstartText_1011_h3 Step-by-Step @quickstartText_1012_h4 Installation @quickstartText_1013_p Install the software using the Windows Installer (if you did not yet do that). @quickstartText_1014_h4 Start the Console @quickstartText_1015_p Click Start , All Programs , H2 , and H2 Console (Command Line) : @quickstartText_1016_p A new console window appears: @quickstartText_1017_p Also, a new browser page should open with the URL http://localhost:8082. You may get a security warning from the firewall. If you don't want other computers in the network to access the database on your machine, you can let the firewall block these connections. Only local connections are required at this time. @quickstartText_1018_h4 Login @quickstartText_1019_p Select Generic H2 and click Connect : @quickstartText_1020_p You are now logged in. @quickstartText_1021_h4 Sample @quickstartText_1022_p Click on the Sample SQL Script : @quickstartText_1023_p The SQL commands appear in the command area. @quickstartText_1024_h4 Execute @quickstartText_1025_p Click Run : @quickstartText_1026_p On the left side, a new entry TEST is added below the database icon. The operations and results of the statements are shown below the script. @quickstartText_1027_h4 Disconnect @quickstartText_1028_p Click on Disconnect : @quickstartText_1029_p to close the database. @quickstartText_1030_h4 End @quickstartText_1031_p Close the console window. For more information, see the Tutorial . @search_1000_b Search: @search_1001_td Highlight keyword(s) @search_1002_a Home @search_1003_a Quickstart @search_1004_a Installation @search_1005_a Tutorial @search_1006_a Features @search_1007_a Performance @search_1008_a Advanced Topics @search_1009_b Reference @search_1010_a SQL Grammar @search_1011_a Functions @search_1012_a Data Types @search_1013_a Javadoc JDBC API @search_1014_a Documentation as PDF @search_1015_b Appendix @search_1016_a Build @search_1017_a History and Roadmap @search_1018_a FAQ and Known Bugs @search_1019_a License @tutorial_1000_h1 Tutorial @tutorial_1001_a Starting and Using the H2 Console @tutorial_1002_a Connecting to a Database using JDBC @tutorial_1003_a Creating New Databases @tutorial_1004_a Using the Server @tutorial_1005_a Using Hibernate @tutorial_1006_a Using Databases in Web Applications @tutorial_1007_a CSV (Comma Separated Values) Support @tutorial_1008_a Upgrade, Backup, and Restore @tutorial_1009_a Using OpenOffice Base @tutorial_1010_a Java Web Start / JNLP @tutorial_1011_a Fulltext Search @tutorial_1012_h2 Starting and Using the H2 Console @tutorial_1013_p This application lets you access a SQL database using a browser interface. This can be a H2 database, or another database that supports the JDBC API. @tutorial_1014_p This is a client / server application, so both a server and a client (a browser) are required to run it. @tutorial_1015_p Depending on your platform and environment, there are multiple ways to start the application: @tutorial_1016_th OS @tutorial_1017_th Start @tutorial_1018_td Windows @tutorial_1019_td Click [Start], [All Programs], [H2], and [H2 Console (Command Line)] @tutorial_1020_td When using the Sun JDK 1.4 or 1.5, a window with the title 'H2 Console ' should appear. When using the Sun JDK 1.6, an icon will be added to the system tray: @tutorial_1021_td If you don't get the window and the system tray icon, then maybe Java is not installed correctly (in this case, try another way to start the application). A browser window should open and point to the Login page http://localhost:8082). @tutorial_1022_td Windows @tutorial_1023_td Open a file browser, navigate to h2/bin, and double click on h2.bat. @tutorial_1024_td A console window appears. If there is a problem, you will see an error message in this window. A browser window will open and point to the Login page (URL: http://localhost:8082). @tutorial_1025_td Any @tutorial_1026_td Open a console window, navigate to the directory 'h2/bin' and type: @tutorial_1027_h3 Firewall @tutorial_1028_p If you start the server, you may get a security warning from the firewall (if you have installed one). If you don't want other computers in the network to access the application on your machine, you can let the firewall block those connections. The connection from the local machine will still work. Only if you want other computers to access the database on this computer, you need allow remote connections in the firewall. @tutorial_1029_p A small firewall is already built into the server: other computers may not connect to the server by default. To change this, go to 'Preferences' and select 'Allow connections from other computers'. @tutorial_1030_h3 Native Version @tutorial_1031_p The native version does not require Java, because it is compiled using GCJ. However H2 does currently not run stable with GCJ on Windows It is possible to compile the software to different platforms. @tutorial_1032_h3 Testing Java @tutorial_1033_p To check the Java version you have installed, open a command prompt and type: @tutorial_1034_p If you get an error message, you may need to add the Java binary directory to the path environment variable. @tutorial_1035_h3 Error Message 'Port is in use' @tutorial_1036_p You can only start one instance of the H2 Console, otherwise you will get the following error message: Port is in use, maybe another ... server already running on... . It is possible to start multiple console applications on the same computer (using different ports), but this is usually not required as the console supports multiple concurrent connections. @tutorial_1037_h3 Using another Port @tutorial_1038_p If the port is in use by another application, you may want to start the H2 Console on a different port. This can be done by changing the port in the file .h2.server.properties. This file is stored in the user directory (for Windows, this is usually in "Documents and Settings/<username>"). The relevant entry is webPort. @tutorial_1039_h3 Starting Successfully @tutorial_1040_p If starting the server from a console window was successful, a new window will open and display the following text: @tutorial_1041_p Don't click inside this window; otherwise you might block the application (if you have the Fast-Edit mode enabled). @tutorial_1042_h3 Connecting to the Server using a Browser @tutorial_1043_p If the server started successfully, you can connect to it using a web browser. The browser needs to support JavaScript, frames and cascading stylesheets (css). If you started the server on the same computer as the browser, go to http://localhost:8082 in the browser. If you want to connect to the application from another computer, you need to provide the IP address of the server, for example: http://192.168.0.2:8082. If you enabled SSL on the server side, the URL needs to start with HTTPS. @tutorial_1044_h3 Multiple Concurrent Sessions @tutorial_1045_p Multiple concurrent browser sessions are supported. As that the database objects reside on the server, the amount of concurrent work is limited by the memory available to the server application. @tutorial_1046_h3 Application Properties @tutorial_1047_p Starting the server will create a configuration file in you local home directory called .h2.server.properties . For Windows installations, this file will be in the directory C:\Documents and Settings\[username] . This file contains the settings of the application. @tutorial_1048_h3 Login @tutorial_1049_p At the login page, you need to provide connection information to connect to a database. Set the JDBC driver class of your database, the JDBC URL, user name and password. If you are done, click [Connect]. @tutorial_1050_p You can save and reuse previously saved settings. The settings are stored in the Application Properties file. @tutorial_1051_h3 Error Messages @tutorial_1052_p Error messages in are shown in red. You can show/hide the stack trace of the exception by clicking on the message. @tutorial_1053_h3 Adding Database Drivers @tutorial_1054_p Additional database drivers can be registered by adding the Jar file location of the driver to the environment variables H2DRIVERS or CLASSPATH. Example (Windows): To add the database driver library C:\Programs\hsqldb\lib\hsqldb.jar, set the environment variable H2DRIVERS to C:\Programs\hsqldb\lib\hsqldb.jar. @tutorial_1055_p Multiple drivers can be set; each entry needs to be separated with a ';' (Windows) or ':' (other operating systems). Spaces in the path names are supported. The settings must not be quoted. @tutorial_1056_p Only the Java version supports additional drivers (this feature is not supported by the Native version). @tutorial_1057_h3 Using the Application @tutorial_1058_p The application has three main panels, the toolbar on top, the tree on the left and the query / result panel on the right. The database objects (for example, tables) are listed on the left panel. Type in a SQL command on the query panel and click 'Run'. The result of the command appears just below the command. @tutorial_1059_h3 Inserting Table Names or Column Names @tutorial_1060_p The table name and column names can be inserted in the script by clicking them in the tree. If you click on a table while the query is empty, a 'SELECT * FROM ...' is added as well. While typing a query, the table that was used is automatically expanded in the tree. For, example if you type 'SELECT * FROM TEST T WHERE T.' then the table TEST is automatically expanded in the tree. @tutorial_1061_h3 Disconnecting and Stopping the Application @tutorial_1062_p On the browser, click 'Disconnect' on the toolbar panel. You will be logged out of the database. However, the server is still running and ready to accept new sessions. @tutorial_1063_p To stop the server, right click on the system tray icon and select [Exit]. If you don't have the icon (because you started it in another way), press [Ctrl]+[C] on the console where the server was started (Windows), or close the console window. @tutorial_1064_h2 Connecting to a Database using JDBC @tutorial_1065_p To connect to a database, a Java application first needs to load the database driver, and then get a connection. A simple way to do that is using the following code: @tutorial_1066_p This code first loads the driver ( Class.forName() ) and then opens a connection (using DriverManager.getConnection() ). The driver name is "org.h2.Driver" in every case. The database URL always needs to start with jdbc:h2: to be recognized by this database. The second parameter in the getConnection() call is the user name ('sa' for System Administrator in this example). The third parameter is the password. Please note that in this database, user names are not case sensitive, but passwords are case sensitive. @tutorial_1067_h2 Creating New Databases @tutorial_1068_p By default, if the database specified in the URL does not yet exist, a new (empty) database is created automatically. The user that created the database automatically becomes the administrator of this database. @tutorial_1069_h2 Using the Server @tutorial_1070_p H2 currently supports three servers: a Web Server, a TCP Server and an ODBC Server. The servers can be started in different ways. @tutorial_1071_h3 Starting the Server from Command Line @tutorial_1072_p To start the Server from the command line with the default settings, run @tutorial_1073_p This will start the Server with the default options. To get the list of options and default values, run @tutorial_1074_p There are options available to use a different ports, and start or not start parts of the Server and so on. For details, see the API documentation of the Server tool. @tutorial_1075_h3 Connecting to the TCP Server @tutorial_1076_p To remotly connect to a database using the TCP server, use the following driver and database URL: @tutorial_1077_li JDBC driver class: org.h2.Driver @tutorial_1078_li Database URL: jdbc:h2:tcp://localhost/~/test @tutorial_1079_p For details about the database URL, see also in Features. @tutorial_1080_h3 Starting the Server within an Application @tutorial_1081_p It is also possible to start and stop a Server from within an application. Sample code: @tutorial_1082_h3 Stopping a TCP Server from Another Process @tutorial_1083_p The TCP Server can be stopped from another process. To stop the server from the command line, run: @tutorial_1084_p To stop the server from a user application, use the following code: @tutorial_1085_p This function will call System.exit on the server. This function should be called after all connection to the databases are closed to avoid recovery when the databases are opened the next time. To stop remote server, remote connections must be enabled on the server. @tutorial_1086_h3 Limitations of the Server @tutorial_1087_p There currently are a few limitations when using the server or cluster mode: @tutorial_1088_li Statement.cancel() is only supported in embedded mode. A connection can only execute one operation at a time in server or cluster mode, and is blocked until this operation is finished. @tutorial_1089_h2 Using Hibernate @tutorial_1090_p This database supports Hibernate version 3.1 and newer. You can use the HSQLDB Dialect, or the native H2 Dialect that is available in the file src/tools/org/h2/tools/hibernate/H2Dialect.txt. The H2 dialect is included in newer version of Hibernate. For versions where the dialect is missing, you need to copy the file into the folder src\org\hibernate\dialect (Hibernate 3.1), rename it to H2Dialect.java and re-compile hibernate. @tutorial_1091_h2 Using Databases in Web Applications @tutorial_1092_p There are multiple ways to access a database from within web applications. Here are some examples if you use Tomcat or JBoss. @tutorial_1093_h3 Embedded Mode @tutorial_1094_p The (currently) most simple solution is to use the database in the embedded mode, that means open a connection in your application when it starts (a good solution is using a Servlet Listener, see below), or when a session starts. A database can be accessed from multiple sessions and applications at the same time, as long as they run in the same process. Most Servlet Containers (for example Tomcat) are just using one process, so this is not a problem (unless you run Tomcat in clustered mode). Tomcat uses multiple threads and multiple classloaders. If multiple applications access the same database at the same time, you need to put the database jar in the shared/lib or server/lib directory. It is a good idea to open the database when the web application starts, and close it when the web applications stops. If using multiple applications, only one (any) of them needs to do that. In the application, an idea is to use one connection per Session, or even one connection per request (action). Those connections should be closed after use if possible (but it's not that bad if they don't get closed). @tutorial_1095_h3 Server Mode @tutorial_1096_p The server mode is similar, but it allows you to run the server in another process. @tutorial_1097_h3 Using a Servlet Listener to Start and Stop a Database @tutorial_1098_p Add the h2.jar file your web application, and add the following snippet to your web.xml file (after context-param and before filter): @tutorial_1099_p For details on how to access the database, see the code DbStarter.java @tutorial_1100_h2 CSV (Comma Separated Values) Support @tutorial_1101_p The CSV file support can be used inside the database using the functions CSVREAD and CSVWRITE, and the CSV library can be used outside the database as a standalone tool. @tutorial_1102_h3 Writing a CSV File from Within a Database @tutorial_1103_p The built-in function CSVWRITE can be used to create a CSV file from a query. Example: @tutorial_1104_h3 Reading a CSV File from Within a Database @tutorial_1105_p A CSV file can be read using the function CSVREAD. Example: @tutorial_1106_h3 Writing a CSV File from a Java Application @tutorial_1107_p The CSV tool can be used in a Java application even when not using a database at all. Example: @tutorial_1108_h3 Reading a CSV File from a Java Application @tutorial_1109_p It is possible to read a CSV file without opening a database. Example: @tutorial_1110_h2 Upgrade, Backup, and Restore @tutorial_1111_h3 Database Upgrade @tutorial_1112_p The recommended way to upgrade from one version of the database engine to the next version is to create a backup of the database (in the form of a SQL script) using the old engine, and then execute the SQL script using the new engine. @tutorial_1113_h3 Backup using the Script Tool @tutorial_1114_p There are different ways to backup a database. For example, it is possible to copy the database files. However, this is not recommended while the database is in use. Also, the database files are not human readable and quite large. The recommended way to backup a database is to create a compressed SQL script file. This can be done using the Script tool: @tutorial_1115_p It is also possible to use the SQL command SCRIPT to create the backup of the database. For more information about the options, see the SQL command SCRIPT. The backup can be done remotely, however the file will be created on the server side. The built in FTP server could be used to retrieve the file from the server. @tutorial_1116_h3 Restore from a Script @tutorial_1117_p To restore a database from a SQL script file, you can use the RunScript tool: @tutorial_1118_p For more information about the options, see the SQL command RUNSCRIPT. The restore can be done remotely, however the file needs to be on the server side. The built in FTP server could be used to copy the file to the server. It is also possible to use the SQL command RUNSCRIPT to execute a SQL script. SQL script files may contain references to other script files, in the form of RUNSCRIPT commands. However, when using the server mode, the references script files need to be available on the server side. @tutorial_1119_h3 Online Backup @tutorial_1120_p The BACKUP SQL statement and the Backup tool both create a zip file with all database files. However, the contents of this file are not human readable. Other than the SCRIPT statement, the BACKUP statement does not lock the database objects, and therefore does not block other users. The resulting backup is transactionally consistent: @tutorial_1121_p The Backup tool (org.h2.tools.Backup) can not be used to create a online backup; the database must not be in use while running this program. @tutorial_1122_h2 Using OpenOffice Base @tutorial_1123_p OpenOffice.org Base supports database access over the JDBC API. To connect to a H2 database using OpenOffice Base, you first need to add the JDBC driver to OpenOffice. The steps to connect to a H2 database are: @tutorial_1124_li Stop OpenOffice, including the autostart @tutorial_1125_li Copy h2.jar into the directory <OpenOffice>\program\classes @tutorial_1126_li Start OpenOffice Base @tutorial_1127_li Connect to an existing database, select JDBC, [Next] @tutorial_1128_li Example datasource URL: jdbc:h2:c:/temp/test @tutorial_1129_li JDBC driver class: org.h2.Driver @tutorial_1130_p Now you can access the database stored in the directory C:/temp. @tutorial_1131_h2 Java Web Start / JNLP @tutorial_1132_p When using Java Web Start / JNLP (Java Network Launch Protocol), permissions tags must be set in the .jnlp file, and the application .jar file must be signed. Otherwise, when trying to write to the file system, the following exception will occur: java.security.AccessControlException: access denied (java.io.FilePermission ... read). Example permission tags: @tutorial_1133_h2 Fulltext Search @tutorial_1134_p H2 supports Lucene full text search and native full text search implementation. @tutorial_1135_h3 Using the Native Full Text Search @tutorial_1136_p To initialize, call: @tutorial_1137_p Afterwards, you can create a full text index for a table using: @tutorial_1138_p PUBLIC is the schema, TEST is the table name. The list of column names (column separated) is optional, in this case all columns are indexed. The index is updated in read time. To search the index, use the following query: @tutorial_1139_p You can also call the index from within a Java application: @tutorial_1140_h3 Using the Lucene Fulltext Search @tutorial_1141_p To use the Lucene full text search, you first need to rename the file FullTextLucene.java.txt to FullTestLucene.java and compile it. Also, you need the Lucene library in the classpath. To initialize, call: @tutorial_1142_p Afterwards, you can create a full text index for a table using: @tutorial_1143_p PUBLIC is the schema, TEST is the table name. The list of column names (column separated) is optional, in this case all columns are indexed. The index is updated in read time. To search the index, use the following query: @tutorial_1144_p You can also call the index from within a Java application: