TCP Server: result sets below this size are sent in one block
The default result set fetch size when using the server mode
@advanced_1330_h2
@advanced_1330_h2
Glossary and Links
Glossary and Links
...
@@ -1646,7 +1646,7 @@ MySQL
...
@@ -1646,7 +1646,7 @@ MySQL
PostgreSQL
PostgreSQL
@features_1090_td
@features_1090_td
Embedded Mode (Java)
Pure Java
@features_1091_td
@features_1091_td
Yes
Yes
...
@@ -1664,7 +1664,7 @@ No
...
@@ -1664,7 +1664,7 @@ No
No
No
@features_1096_td
@features_1096_td
Pure Java
Embedded Mode (Java)
@features_1097_td
@features_1097_td
Yes
Yes
...
@@ -1700,25 +1700,25 @@ N/A
...
@@ -1700,25 +1700,25 @@ N/A
N/A
N/A
@features_1108_td
@features_1108_td
Transaction Isolation
In-Memory Mode
@features_1109_td
@features_1109_td
Yes
Yes
@features_1110_td
@features_1110_td
Yes
No
@features_1111_td
@features_1111_td
No
Yes
@features_1112_td
@features_1112_td
Yes
No
@features_1113_td
@features_1113_td
Yes
No
@features_1114_td
@features_1114_td
Cost Based Optimizer
Transaction Isolation
@features_1115_td
@features_1115_td
Yes
Yes
...
@@ -1736,13 +1736,13 @@ Yes
...
@@ -1736,13 +1736,13 @@ Yes
Yes
Yes
@features_1120_td
@features_1120_td
Clustering
Cost Based Optimizer
@features_1121_td
@features_1121_td
Yes
Yes
@features_1122_td
@features_1122_td
No
Yes
@features_1123_td
@features_1123_td
No
No
...
@@ -1754,936 +1754,1008 @@ Yes
...
@@ -1754,936 +1754,1008 @@ Yes
Yes
Yes
@features_1126_td
@features_1126_td
Encrypted Database
Clustering
@features_1127_td
@features_1127_td
Yes
Yes
@features_1128_td
@features_1128_td
Yes
No
@features_1129_td
@features_1129_td
No
No
@features_1130_td
@features_1130_td
No
Yes
@features_1131_td
@features_1131_td
No
Yes
@features_1132_td
@features_1132_td
Files per Database
Encrypted Database
@features_1133_td
@features_1133_td
Few
Yes
@features_1134_td
@features_1134_td
Many
Yes
@features_1135_td
@features_1135_td
Few
No
@features_1136_td
@features_1136_td
Many
No
@features_1137_td
@features_1137_td
Many
No
@features_1138_td
@features_1138_td
Footprint (jar/dll size)
ODBC Driver
@features_1139_td
@features_1139_td
~ 1 MB
Yes
@features_1140_td
@features_1140_td
~ 2 MB
Yes?
@features_1141_td
@features_1141_td
~ 600 KB
No
@features_1142_td
@features_1142_td
~ 4 MB
Yes
@features_1143_td
@features_1143_td
Yes
@features_1144_td
Fulltext Search
@features_1145_td
Yes
@features_1146_td
No
@features_1147_td
No
@features_1148_td
Yes
@features_1149_td
Yes
@features_1150_td
User Defined Datatypes
@features_1151_td
Yes
@features_1152_td
No
@features_1153_td
No
@features_1154_td
Yes
@features_1155_td
Yes
@features_1156_td
Files per Database
@features_1157_td
Few
@features_1158_td
Many
@features_1159_td
Few
@features_1160_td
Many
@features_1161_td
Many
@features_1162_td
Footprint (jar/dll size)
@features_1163_td
~ 1 MB
@features_1164_td
~ 2 MB
@features_1165_td
~ 600 KB
@features_1166_td
~ 4 MB
@features_1167_td
~ 6 MB
~ 6 MB
@features_1144_h3
@features_1168_h3
Derby and HSQLDB
Derby and HSQLDB
@features_1145_p
@features_1169_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').
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
@features_1170_h3
DaffodilDb and One$Db
DaffodilDb and One$Db
@features_1147_p
@features_1171_p
It looks like the development of this database has stopped. The last release was February 2006.
It looks like the development of this database has stopped. The last release was February 2006.
@features_1148_h3
@features_1172_h3
McKoi
McKoi
@features_1149_p
@features_1173_p
It looks like the development of this database has stopped. The last release was August 2004
It looks like the development of this database has stopped. The last release was August 2004
@features_1150_h2
@features_1174_h2
H2 in Use
H2 in Use
@features_1151_p
@features_1175_p
For a list of applications that work with or use H2, see: <a href="http://groups.google.com/group/h2-database/web/h2-in-use">http://groups.google.com/group/h2-database/web/h2-in-use</a>
For a list of applications that work with or use H2, see: <a href="http://groups.google.com/group/h2-database/web/h2-in-use">http://groups.google.com/group/h2-database/web/h2-in-use</a>
@features_1152_h2
@features_1176_h2
Connection Modes
Connection Modes
@features_1153_p
@features_1177_p
The following connection modes are supported:
The following connection modes are supported:
@features_1154_li
@features_1178_li
Local connections using JDBC (embedded)
Local connections using JDBC (embedded)
@features_1155_li
@features_1179_li
Remote connections using JDBC over TCP/IP (client/server)
Remote connections using JDBC over TCP/IP (client/server)
@features_1156_li
@features_1180_li
Remote connections using ODBC over TCP/IP (client/server)
Remote connections using ODBC over TCP/IP (client/server)
@features_1157_li
@features_1181_li
In-Memory databases (private and shared)
In-Memory databases (private and shared)
@features_1158_h2
@features_1182_h2
Database URL Overview
Database URL Overview
@features_1159_p
@features_1183_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.
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.
The database URL for connecting to a local database is <code>jdbc:h2:[file:][<path>]<databaseName></code> . The prefix <code>file:</code> 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.
The database URL for connecting to a local database is <code>jdbc:h2:[file:][<path>]<databaseName></code> . The prefix <code>file:</code> 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
@features_1239_h2
Memory-Only Databases
Memory-Only Databases
@features_1216_p
@features_1240_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.
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
@features_1241_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 <code>jdbc:h2:mem:</code> Opening two connections within the same virtual machine means opening two different (private) databases.
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 <code>jdbc:h2:mem:</code> Opening two connections within the same virtual machine means opening two different (private) databases.
@features_1218_p
@features_1242_p
Sometimes multiple connections to the same memory-only database are required. In this case, the database URL must include a name. Example: <code>jdbc:h2:mem:db1</code> . Accessing the same database in this way only works within the same virtual machine and class loader environment.
Sometimes multiple connections to the same memory-only database are required. In this case, the database URL must include a name. Example: <code>jdbc:h2:mem:db1</code> . Accessing the same database in this way only works within the same virtual machine and class loader environment.
@features_1219_p
@features_1243_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: <code>jdbc:h2:tcp://localhost/mem:db1</code> (using private database remotely is also possible).
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: <code>jdbc:h2:tcp://localhost/mem:db1</code> (using private database remotely is also possible).
@features_1220_p
@features_1244_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
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
@features_1245_h2
Connecting to a Database with File Encryption
Connecting to a Database with File Encryption
@features_1222_p
@features_1246_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:
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
@features_1247_h2
Database File Locking
Database File Locking
@features_1224_p
@features_1248_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.
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
@features_1249_p
The following file locking methods are implemented:
The following file locking methods are implemented:
@features_1226_li
@features_1250_li
The default method is 'file' and uses a watchdog thread to protect the database file. The watchdog reads the lock file each second.
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
@features_1251_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.
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
@features_1252_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.
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
@features_1253_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:
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
@features_1254_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:
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
@features_1255_p
For more information about the algorithms please see in Advanced Topics under File Locking Protocol.
For more information about the algorithms please see in Advanced Topics under File Locking Protocol.
@features_1232_h2
@features_1256_h2
Opening a Database Only if it Already Exists
Opening a Database Only if it Already Exists
@features_1233_p
@features_1257_p
By default, when an application calls <code>DriverManager.getConnection(url,...)</code> 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 <code>;ifexists=true</code> 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:
By default, when an application calls <code>DriverManager.getConnection(url,...)</code> 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 <code>;ifexists=true</code> 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
@features_1258_h2
Closing the Database
Closing the Database
@features_1235_h3
@features_1259_h3
Delayed Database Closing
Delayed Database Closing
@features_1236_p
@features_1260_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:
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
@features_1261_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: <code>jdbc:h2:~/test;DB_CLOSE_DELAY=10</code> .
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: <code>jdbc:h2:~/test;DB_CLOSE_DELAY=10</code> .
@features_1238_h3
@features_1262_h3
Don't Close the Database when the VM Exits
Don't Close the Database when the VM Exits
@features_1239_p
@features_1263_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:
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
@features_1264_h2
Log Index Changes
Log Index Changes
@features_1241_p
@features_1265_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.
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
@features_1266_h3
Ignore Unknown Settings
Ignore Unknown Settings
@features_1243_p
@features_1267_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.
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
@features_1268_h3
Changing Other Settings when Opening a Connection
Changing Other Settings when Opening a Connection
@features_1245_p
@features_1269_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 <code>setting=value</code> at the end of an URL is the same as executing the statement <code>SET setting value</code> just after connecting. For a list of settings supported by this database please see the SQL grammar documentation.
In addition to the settings already described (cipher, file_lock, ifexists, user, password), other database settings can be passed in the database URL. Adding <code>setting=value</code> at the end of an URL is the same as executing the statement <code>SET setting value</code> just after connecting. For a list of settings supported by this database please see the SQL grammar documentation.
@features_1246_h2
@features_1270_h2
Custom File Access Mode
Custom File Access Mode
@features_1247_p
@features_1271_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:
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
@features_1272_p
For more information see <a href="advanced.html#durability_problems">Durability Problems</a> . On many operating systems the access mode 'rws' does not guarantee that the data is written to the disk.
For more information see <a href="advanced.html#durability_problems">Durability Problems</a> . On many operating systems the access mode 'rws' does not guarantee that the data is written to the disk.
@features_1249_h2
@features_1273_h2
Multiple Connections
Multiple Connections
@features_1250_h3
@features_1274_h3
Opening Multiple Databases at the Same Time
Opening Multiple Databases at the Same Time
@features_1251_p
@features_1275_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.
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
@features_1276_h3
Multiple Connections to the Same Database: Client/Server
Multiple Connections to the Same Database: Client/Server
@features_1253_p
@features_1277_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).
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
@features_1278_h3
Multithreading Support
Multithreading Support
@features_1255_p
@features_1279_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.
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
@features_1280_h3
Locking, Lock-Timeout, Deadlocks
Locking, Lock-Timeout, Deadlocks
@features_1257_p
@features_1281_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.
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
@features_1282_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:
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
@features_1283_th
Type of Lock
Type of Lock
@features_1260_th
@features_1284_th
SQL Statement
SQL Statement
@features_1261_td
@features_1285_td
Read
Read
@features_1262_td
@features_1286_td
SELECT * FROM TEST
SELECT * FROM TEST
@features_1263_td
@features_1287_td
CALL SELECT MAX(ID) FROM TEST
CALL SELECT MAX(ID) FROM TEST
@features_1264_td
@features_1288_td
SCRIPT
SCRIPT
@features_1265_td
@features_1289_td
Write
Write
@features_1266_td
@features_1290_td
SELECT * FROM TEST WHERE 1=0 FOR UPDATE
SELECT * FROM TEST WHERE 1=0 FOR UPDATE
@features_1267_td
@features_1291_td
Write
Write
@features_1268_td
@features_1292_td
INSERT INTO TEST VALUES(1, 'Hello')
INSERT INTO TEST VALUES(1, 'Hello')
@features_1269_td
@features_1293_td
INSERT INTO TEST SELECT * FROM TEST
INSERT INTO TEST SELECT * FROM TEST
@features_1270_td
@features_1294_td
UPDATE TEST SET NAME='Hi'
UPDATE TEST SET NAME='Hi'
@features_1271_td
@features_1295_td
DELETE FROM TEST
DELETE FROM TEST
@features_1272_td
@features_1296_td
Write
Write
@features_1273_td
@features_1297_td
ALTER TABLE TEST ...
ALTER TABLE TEST ...
@features_1274_td
@features_1298_td
CREATE INDEX ... ON TEST ...
CREATE INDEX ... ON TEST ...
@features_1275_td
@features_1299_td
DROP INDEX ...
DROP INDEX ...
@features_1276_p
@features_1300_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.
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
@features_1301_h2
Database File Layout
Database File Layout
@features_1278_p
@features_1302_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:
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
@features_1303_th
File Name
File Name
@features_1280_th
@features_1304_th
Description
Description
@features_1281_th
@features_1305_th
Number of Files
Number of Files
@features_1282_td
@features_1306_td
test.data.db
test.data.db
@features_1283_td
@features_1307_td
Data file
Data file
@features_1284_td
@features_1308_td
Contains the data for all tables
Contains the data for all tables
@features_1285_td
@features_1309_td
Format: <database>.data.db
Format: <database>.data.db
@features_1286_td
@features_1310_td
1 per database
1 per database
@features_1287_td
@features_1311_td
test.index.db
test.index.db
@features_1288_td
@features_1312_td
Index file
Index file
@features_1289_td
@features_1313_td
Contains the data for all (btree) indexes
Contains the data for all (btree) indexes
@features_1290_td
@features_1314_td
Format: <database>.index.db
Format: <database>.index.db
@features_1291_td
@features_1315_td
1 per database
1 per database
@features_1292_td
@features_1316_td
test.0.log.db
test.0.log.db
@features_1293_td
@features_1317_td
Log file
Log file
@features_1294_td
@features_1318_td
The log file is used for recovery
The log file is used for recovery
@features_1295_td
@features_1319_td
Format: <database>.<id>.log.db
Format: <database>.<id>.log.db
@features_1296_td
@features_1320_td
0 or more per database
0 or more per database
@features_1297_td
@features_1321_td
test.lock.db
test.lock.db
@features_1298_td
@features_1322_td
Database lock file
Database lock file
@features_1299_td
@features_1323_td
Exists only if the database is open
Exists only if the database is open
@features_1300_td
@features_1324_td
Format: <database>.lock.db
Format: <database>.lock.db
@features_1301_td
@features_1325_td
1 per database
1 per database
@features_1302_td
@features_1326_td
test.trace.db
test.trace.db
@features_1303_td
@features_1327_td
Trace file
Trace file
@features_1304_td
@features_1328_td
Contains trace information
Contains trace information
@features_1305_td
@features_1329_td
Format: <database>.trace.db
Format: <database>.trace.db
@features_1306_td
@features_1330_td
If the file is too big, it is renamed to <database>.trace.db.old
If the file is too big, it is renamed to <database>.trace.db.old
Database name and location are not stored inside the database names.
Database name and location are not stored inside the database names.
@features_1325_p
@features_1349_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).
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
@features_1350_p
As there is no platform specific data in the files, they can be moved to other operating systems without problems.
As there is no platform specific data in the files, they can be moved to other operating systems without problems.
@features_1327_h3
@features_1351_h3
Backup
Backup
@features_1328_p
@features_1352_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.
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
@features_1353_p
To backup data while the database is running, the SQL command SCRIPT can be used.
To backup data while the database is running, the SQL command SCRIPT can be used.
@features_1330_h2
@features_1354_h2
Logging and Recovery
Logging and Recovery
@features_1331_p
@features_1355_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.
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
@features_1356_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.
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
@features_1357_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.
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
@features_1358_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.
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
@features_1359_h2
Compatibility
Compatibility
@features_1336_p
@features_1360_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:
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
@features_1361_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).
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
@features_1362_h3
Compatibility Modes
Compatibility Modes
@features_1339_p
@features_1363_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 <code>jdbc:h2:~/test;MODE=HSQLDB</code> or the SQL statement <code>SET MODE HSQLDB</code> . Here is the list of currently supported modes and the difference to the regular mode:
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 <code>jdbc:h2:~/test;MODE=HSQLDB</code> or the SQL statement <code>SET MODE HSQLDB</code> . Here is the list of currently supported modes and the difference to the regular mode:
@features_1340_th
@features_1364_th
Mode
Mode
@features_1341_th
@features_1365_th
Differences
Differences
@features_1342_td
@features_1366_td
PostgreSQL
PostgreSQL
@features_1343_td
@features_1367_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.
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
@features_1368_td
MySQL
MySQL
@features_1345_td
@features_1369_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.
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
@features_1370_td
HSQLDB
HSQLDB
@features_1347_td
@features_1371_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.
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
@features_1372_h2
Using the Trace Options
Using the Trace Options
@features_1349_p
@features_1373_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:
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
@features_1374_li
Trace to System.out and/or a file
Trace to System.out and/or a file
@features_1351_li
@features_1375_li
Support for trace levels OFF, ERROR, INFO, and DEBUG
Support for trace levels OFF, ERROR, INFO, and DEBUG
@features_1352_li
@features_1376_li
The maximum size of the trace file can be set
The maximum size of the trace file can be set
@features_1353_li
@features_1377_li
The Java code generation is possible
The Java code generation is possible
@features_1354_li
@features_1378_li
Trace can be enabled at runtime by manually creating a file
Trace can be enabled at runtime by manually creating a file
@features_1355_h3
@features_1379_h3
Trace Options
Trace Options
@features_1356_p
@features_1380_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:
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
@features_1381_p
The trace level can be changed at runtime by executing the SQL command <code>SET TRACE_LEVEL_SYSTEM_OUT level</code> (for System.out tracing) or <code>SET TRACE_LEVEL_FILE level</code> (for file tracing). Example:
The trace level can be changed at runtime by executing the SQL command <code>SET TRACE_LEVEL_SYSTEM_OUT level</code> (for System.out tracing) or <code>SET TRACE_LEVEL_FILE level</code> (for file tracing). Example:
@features_1358_h3
@features_1382_h3
Setting the Maximum Size of the Trace File
Setting the Maximum Size of the Trace File
@features_1359_p
@features_1383_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 <code>SET TRACE_MAX_FILE_SIZE maximumFileSizeInMB</code> . 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:
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 <code>SET TRACE_MAX_FILE_SIZE maximumFileSizeInMB</code> . 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
@features_1384_h3
Java Code Generation
Java Code Generation
@features_1361_p
@features_1385_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:
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
@features_1386_p
You need to filter out the lines without /**/ to get the Java source code. In Windows, a simple way to do that is:
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
@features_1387_p
Afterwards, you need to complete the file Trace.java before it can be compiled, for example with:
Afterwards, you need to complete the file Trace.java before it can be compiled, for example with:
@features_1364_p
@features_1388_p
Also, the user name and password needs to be set, because they are not listed in the trace file.
Also, the user name and password needs to be set, because they are not listed in the trace file.
@features_1365_h3
@features_1389_h3
Enabling the Trace Option at Runtime by Manually Creating a File
Enabling the Trace Option at Runtime by Manually Creating a File
@features_1366_p
@features_1390_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.
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
@features_1391_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.
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
@features_1392_h2
Read Only Databases
Read Only Databases
@features_1369_p
@features_1393_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().
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
@features_1394_h2
Read Only Databases in Zip or Jar File
Read Only Databases in Zip or Jar File
@features_1371_p
@features_1395_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:
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
@features_1396_p
Afterwards, you can log out, and directly open the database in the zip file using the following database URL:
Afterwards, you can log out, and directly open the database in the zip file using the following database URL:
@features_1373_p
@features_1397_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.
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
@features_1398_h2
Binary and Text Storage Formats
Binary and Text Storage Formats
@features_1375_p
@features_1399_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
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
@features_1400_h2
Graceful Handling of Low Disk Space Situations
Graceful Handling of Low Disk Space Situations
@features_1377_p
@features_1401_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.
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
@features_1402_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.
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
@features_1403_h3
Opening a Corrupted Database
Opening a Corrupted Database
@features_1380_p
@features_1404_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.
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
@features_1405_h2
Computed Columns / Function Based Index
Computed Columns / Function Based Index
@features_1382_p
@features_1406_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:
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
@features_1407_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:
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
@features_1408_h2
Multi-Dimensional Indexes
Multi-Dimensional Indexes
@features_1385_p
@features_1409_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.
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
@features_1410_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).
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
@features_1411_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.
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
@features_1412_h2
Using Passwords
Using Passwords
@features_1389_h3
@features_1413_h3
Using Secure Passwords
Using Secure Passwords
@features_1390_p
@features_1414_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:
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
@features_1415_p
i'sE2rtPiUKtT (it's easy to remember this password if you know the trick)
i'sE2rtPiUKtT (it's easy to remember this password if you know the trick)
@features_1392_h3
@features_1416_h3
Passwords: Using Char Arrays instead of Strings
Passwords: Using Char Arrays instead of Strings
@features_1393_p
@features_1417_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).
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
@features_1418_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.
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
@features_1419_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:
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
@features_1420_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).
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
@features_1421_h3
Passing the User Name and/or Password in the URL
Passing the User Name and/or Password in the URL
@features_1398_p
@features_1422_p
Instead of passing the user name as a separate parameter as in <code>Connection conn = DriverManager. getConnection("jdbc:h2:~/test", "sa", "123");</code> the user name (and/or password) can be supplied in the URL itself: <code>Connection conn = DriverManager. getConnection("jdbc:h2:~/test;USER=sa;PASSWORD=123");</code> The settings in the URL override the settings passed as a separate parameter.
Instead of passing the user name as a separate parameter as in <code>Connection conn = DriverManager. getConnection("jdbc:h2:~/test", "sa", "123");</code> the user name (and/or password) can be supplied in the URL itself: <code>Connection conn = DriverManager. getConnection("jdbc:h2:~/test;USER=sa;PASSWORD=123");</code> The settings in the URL override the settings passed as a separate parameter.
@features_1399_h2
@features_1423_h2
User Defined Functions and Stored Procedures
User Defined Functions and Stored Procedures
@features_1400_p
@features_1424_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:
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
@features_1425_p
The Java function must be registered in the database by calling CREATE ALIAS:
The Java function must be registered in the database by calling CREATE ALIAS:
@features_1402_p
@features_1426_p
For a complete sample application, see src/test/org/h2/samples/Function.java.
For a complete sample application, see src/test/org/h2/samples/Function.java.
@features_1403_h3
@features_1427_h3
Function Data Type Mapping
Function Data Type Mapping
@features_1404_p
@features_1428_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'.
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
@features_1429_h3
Functions that require a Connection
Functions that require a Connection
@features_1406_p
@features_1430_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.
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
@features_1431_h3
Functions throwing an Exception
Functions throwing an Exception
@features_1408_p
@features_1432_p
If a function throws an Exception, then the current statement is rolled back and the exception is thrown to the application.
If a function throws an Exception, then the current statement is rolled back and the exception is thrown to the application.
@features_1409_h3
@features_1433_h3
Functions returning a Result Set
Functions returning a Result Set
@features_1410_p
@features_1434_p
Functions may returns a result set. Such a function can be called with the CALL statement:
Functions may returns a result set. Such a function can be called with the CALL statement:
@features_1411_h3
@features_1435_h3
Using SimpleResultSet
Using SimpleResultSet
@features_1412_p
@features_1436_p
A function that returns a result set can create this result set from scratch using the SimpleResultSet tool:
A function that returns a result set can create this result set from scratch using the SimpleResultSet tool:
@features_1413_h3
@features_1437_h3
Using a Function as a Table
Using a Function as a Table
@features_1414_p
@features_1438_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.
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
@features_1439_h2
Triggers
Triggers
@features_1416_p
@features_1440_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:
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
@features_1441_p
The connection can be used to query or update data in other tables. The trigger then needs to be defined in the database:
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
@features_1442_p
The trigger can be used to veto a change, by throwing a SQL Exception.
The trigger can be used to veto a change, by throwing a SQL Exception.
@features_1419_h2
@features_1443_h2
Compacting a Database
Compacting a Database
@features_1420_p
@features_1444_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:
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
@features_1445_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.
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
@features_1446_h2
Cache Settings
Cache Settings
@features_1423_p
@features_1447_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.
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
@features_1448_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.
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
@features_1449_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.
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
@features_1450_h2
Why Java
Why Java
@features_1427_p
@features_1451_p
A few reasons using a Java database are:
A few reasons using a Java database are:
@features_1428_li
@features_1452_li
Very simple to integrate in Java applications
Very simple to integrate in Java applications
@features_1429_li
@features_1453_li
Support for many different platforms
Support for many different platforms
@features_1430_li
@features_1454_li
More secure than native applications (no buffer overflows)
More secure than native applications (no buffer overflows)
@features_1431_li
@features_1455_li
User defined functions (or triggers) run very fast
User defined functions (or triggers) run very fast
@features_1432_li
@features_1456_li
Unicode support
Unicode support
@features_1433_p
@features_1457_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.
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
@features_1458_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.
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
@features_1459_p
Java is also future proof: A lot of companies support Java, and it is now open source.
Java is also future proof: A lot of companies support Java, and it is now open source.
@features_1436_p
@features_1460_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.
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
@frame_1000_p
...
@@ -5087,7 +5159,7 @@ Using the Native Full Text Search
...
@@ -5087,7 +5159,7 @@ Using the Native Full Text Search
To initialize, call:
To initialize, call:
@tutorial_1137_p
@tutorial_1137_p
Afterwards, you can create a full text index for a table using:
You need to initialize it in each database where you want to use it. Afterwards, you can create a full text index for a table using:
@tutorial_1138_p
@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:
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:
...
@@ -5099,10 +5171,10 @@ You can also call the index from within a Java application:
...
@@ -5099,10 +5171,10 @@ You can also call the index from within a Java application:
Using the Lucene Fulltext Search
Using the Lucene Fulltext Search
@tutorial_1141_p
@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:
To use the Lucene full text search, you need the Lucene library in the classpath. How his is done depends on the application; if you use the H2 Console, you can add the Lucene jar file to the the environment variables H2DRIVERS or CLASSPATH. To initialize the Lucene full text search in a database, call:
@tutorial_1142_p
@tutorial_1142_p
Afterwards, you can create a full text index for a table using:
You need to initialize it in each database where you want to use it. Afterwards, you can create a full text index for a table using:
@tutorial_1143_p
@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:
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:
@@ -325,9 +325,9 @@ advanced_1323_td=Relative or absolute directory where the script files are store
...
@@ -325,9 +325,9 @@ advanced_1323_td=Relative or absolute directory where the script files are store
advanced_1324_td=h2.serverCachedObjects
advanced_1324_td=h2.serverCachedObjects
advanced_1325_td=64
advanced_1325_td=64
advanced_1326_td=TCP Server\:number of cached objects per session
advanced_1326_td=TCP Server\:number of cached objects per session
advanced_1327_td=h2.serverSmallResultSetSize
advanced_1327_td=h2.serverResultSetFetchSize
advanced_1328_td=100
advanced_1328_td=100
advanced_1329_td=TCP Server\:result sets below this size are sent in one block
advanced_1329_td=The default result set fetch size when using the server mode
advanced_1330_h2=Glossary and Links
advanced_1330_h2=Glossary and Links
advanced_1331_th=Term
advanced_1331_th=Term
advanced_1332_th=Description
advanced_1332_th=Description
...
@@ -547,13 +547,13 @@ features_1086_th=Derby
...
@@ -547,13 +547,13 @@ features_1086_th=Derby
features_1087_th=HSQLDB
features_1087_th=HSQLDB
features_1088_th=MySQL
features_1088_th=MySQL
features_1089_th=PostgreSQL
features_1089_th=PostgreSQL
features_1090_td=Embedded Mode (Java)
features_1090_td=Pure Java
features_1091_td=Yes
features_1091_td=Yes
features_1092_td=Yes
features_1092_td=Yes
features_1093_td=Yes
features_1093_td=Yes
features_1094_td=No
features_1094_td=No
features_1095_td=No
features_1095_td=No
features_1096_td=Pure Java
features_1096_td=Embedded Mode (Java)
features_1097_td=Yes
features_1097_td=Yes
features_1098_td=Yes
features_1098_td=Yes
features_1099_td=Yes
features_1099_td=Yes
...
@@ -565,335 +565,359 @@ features_1104_td=Slow
...
@@ -565,335 +565,359 @@ features_1104_td=Slow
features_1105_td=Fast
features_1105_td=Fast
features_1106_td=N/A
features_1106_td=N/A
features_1107_td=N/A
features_1107_td=N/A
features_1108_td=Transaction Isolation
features_1108_td=In-Memory Mode
features_1109_td=Yes
features_1109_td=Yes
features_1110_td=Yes
features_1110_td=No
features_1111_td=No
features_1111_td=Yes
features_1112_td=Yes
features_1112_td=No
features_1113_td=Yes
features_1113_td=No
features_1114_td=Cost Based Optimizer
features_1114_td=Transaction Isolation
features_1115_td=Yes
features_1115_td=Yes
features_1116_td=Yes
features_1116_td=Yes
features_1117_td=No
features_1117_td=No
features_1118_td=Yes
features_1118_td=Yes
features_1119_td=Yes
features_1119_td=Yes
features_1120_td=Clustering
features_1120_td=Cost Based Optimizer
features_1121_td=Yes
features_1121_td=Yes
features_1122_td=No
features_1122_td=Yes
features_1123_td=No
features_1123_td=No
features_1124_td=Yes
features_1124_td=Yes
features_1125_td=Yes
features_1125_td=Yes
features_1126_td=Encrypted Database
features_1126_td=Clustering
features_1127_td=Yes
features_1127_td=Yes
features_1128_td=Yes
features_1128_td=No
features_1129_td=No
features_1129_td=No
features_1130_td=No
features_1130_td=Yes
features_1131_td=No
features_1131_td=Yes
features_1132_td=Files per Database
features_1132_td=Encrypted Database
features_1133_td=Few
features_1133_td=Yes
features_1134_td=Many
features_1134_td=Yes
features_1135_td=Few
features_1135_td=No
features_1136_td=Many
features_1136_td=No
features_1137_td=Many
features_1137_td=No
features_1138_td=Footprint (jar/dll size)
features_1138_td=ODBC Driver
features_1139_td=~ 1 MB
features_1139_td=Yes
features_1140_td=~ 2 MB
features_1140_td=Yes?
features_1141_td=~ 600 KB
features_1141_td=No
features_1142_td=~ 4 MB
features_1142_td=Yes
features_1143_td=~ 6 MB
features_1143_td=Yes
features_1144_h3=Derby and HSQLDB
features_1144_td=Fulltext Search
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_1145_td=Yes
features_1146_h3=DaffodilDb and One$Db
features_1146_td=No
features_1147_p=It looks like the development of this database has stopped. The last release was February 2006.
features_1147_td=No
features_1148_h3=McKoi
features_1148_td=Yes
features_1149_p=It looks like the development of this database has stopped. The last release was August 2004
features_1149_td=Yes
features_1150_h2=H2 in Use
features_1150_td=User Defined Datatypes
features_1151_p=For a list of applications that work with or use H2, see\:<a href\="http\://groups.google.com/group/h2-database/web/h2-in-use">http\://groups.google.com/group/h2-database/web/h2-in-use</a>
features_1151_td=Yes
features_1152_h2=Connection Modes
features_1152_td=No
features_1153_p=The following connection modes are supported\:
features_1153_td=No
features_1154_li=Local connections using JDBC (embedded)
features_1154_td=Yes
features_1155_li=Remote connections using JDBC over TCP/IP (client/server)
features_1155_td=Yes
features_1156_li=Remote connections using ODBC over TCP/IP (client/server)
features_1156_td=Files per Database
features_1157_li=In-Memory databases (private and shared)
features_1157_td=Few
features_1158_h2=Database URL Overview
features_1158_td=Many
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_1169_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_1175_p=For a list of applications that work with or use H2, see\:<a href\="http\://groups.google.com/group/h2-database/web/h2-in-use">http\://groups.google.com/group/h2-database/web/h2-in-use</a>
features_1183_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_1214_p=The database URL for connecting to a local database is <code>jdbc\:h2\:[file\:][<path>]<databaseName></code> . The prefix <code>file\:</code> 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_1214_td=Don't Close the Database when the VM Exits
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_1216_td=User Name and/or Password
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 <code>jdbc\:h2\:mem\:</code> 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\:<code>jdbc\:h2\:mem\:db1</code> . 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\:<code>jdbc\:h2\:tcp\://localhost/mem\:db1</code> (using private database remotely is also possible).
features_1219_td=Log Index Changes
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_1220_td=jdbc\:h2\:<url>;LOG\=2
features_1221_h2=Connecting to a Database with File Encryption
features_1221_td=jdbc\:h2\:file\:~/sample;LOG\=2
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_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_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_1227_td=Custom File Access Mode
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_1229_td=In-Memory (private)
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_1230_td=jdbc\:h2\:mem\:
features_1231_p=For more information about the algorithms please see in Advanced Topics under File Locking Protocol.
features_1231_td=Database in or Zip File
features_1232_h2=Opening a Database Only if it Already Exists
features_1233_p=By default, when an application calls <code>DriverManager.getConnection(url,...)</code> 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 <code>;ifexists\=true</code> 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_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\:<code>jdbc\:h2\:~/test;DB_CLOSE_DELAY\=10</code> .
features_1237_h3=Connecting to an Embedded (Local) Database
features_1238_h3=Don't Close the Database when the VM Exits
features_1238_p=The database URL for connecting to a local database is <code>jdbc\:h2\:[file\:][<path>]<databaseName></code> . The prefix <code>file\:</code> 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_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_1239_h2=Memory-Only Databases
features_1240_h2=Log Index Changes
features_1240_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_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_1241_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 <code>jdbc\:h2\:mem\:</code> Opening two connections within the same virtual machine means opening two different (private) databases.
features_1242_h3=Ignore Unknown Settings
features_1242_p=Sometimes multiple connections to the same memory-only database are required. In this case, the database URL must include a name. Example\:<code>jdbc\:h2\:mem\:db1</code> . Accessing the same database in this way only works within the same virtual machine and class loader environment.
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_1243_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\:<code>jdbc\:h2\:tcp\://localhost/mem\:db1</code> (using private database remotely is also possible).
features_1244_h3=Changing Other Settings when Opening a Connection
features_1244_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_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 <code>setting\=value</code> at the end of an URL is the same as executing the statement <code>SET setting value</code> just after connecting. For a list of settings supported by this database please see the SQL grammar documentation.
features_1245_h2=Connecting to a Database with File Encryption
features_1246_h2=Custom File Access Mode
features_1246_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_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_1247_h2=Database File Locking
features_1248_p=For more information see <a href\="advanced.html\#durability_problems">Durability Problems</a> . On many operating systems the access mode 'rws' does not guarantee that the data is written to the disk.
features_1248_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_1249_h2=Multiple Connections
features_1249_p=The following file locking methods are implemented\:
features_1250_h3=Opening Multiple Databases at the Same Time
features_1250_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_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_1251_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_1252_h3=Multiple Connections to the Same Database\:Client/Server
features_1252_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_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_1253_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_1254_h3=Multithreading Support
features_1254_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_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_1255_p=For more information about the algorithms please see in Advanced Topics under File Locking Protocol.
features_1256_h3=Locking, Lock-Timeout, Deadlocks
features_1256_h2=Opening a Database Only if it Already Exists
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_1257_p=By default, when an application calls <code>DriverManager.getConnection(url,...)</code> 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 <code>;ifexists\=true</code> 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_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_1258_h2=Closing the Database
features_1259_th=Type of Lock
features_1259_h3=Delayed Database Closing
features_1260_th=SQL Statement
features_1260_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_1261_td=Read
features_1261_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\:<code>jdbc\:h2\:~/test;DB_CLOSE_DELAY\=10</code> .
features_1262_td=SELECT * FROM TEST
features_1262_h3=Don't Close the Database when the VM Exits
features_1263_td=CALL SELECT MAX(ID) FROM TEST
features_1263_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_1264_td=SCRIPT
features_1264_h2=Log Index Changes
features_1265_td=Write
features_1265_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_1266_td=SELECT * FROM TEST WHERE 1\=0 FOR UPDATE
features_1266_h3=Ignore Unknown Settings
features_1267_td=Write
features_1267_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_1268_td=INSERT INTO TEST VALUES(1, 'Hello')
features_1268_h3=Changing Other Settings when Opening a Connection
features_1269_td=INSERT INTO TEST SELECT * FROM TEST
features_1269_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 <code>setting\=value</code> at the end of an URL is the same as executing the statement <code>SET setting value</code> just after connecting. For a list of settings supported by this database please see the SQL grammar documentation.
features_1270_td=UPDATE TEST SET NAME\='Hi'
features_1270_h2=Custom File Access Mode
features_1271_td=DELETE FROM TEST
features_1271_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_1272_td=Write
features_1272_p=For more information see <a href\="advanced.html\#durability_problems">Durability Problems</a> . On many operating systems the access mode 'rws' does not guarantee that the data is written to the disk.
features_1273_td=ALTER TABLE TEST ...
features_1273_h2=Multiple Connections
features_1274_td=CREATE INDEX ... ON TEST ...
features_1274_h3=Opening Multiple Databases at the Same Time
features_1275_td=DROP INDEX ...
features_1275_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_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_1276_h3=Multiple Connections to the Same Database\:Client/Server
features_1277_h2=Database File Layout
features_1277_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_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_1278_h3=Multithreading Support
features_1279_th=File Name
features_1279_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_1280_th=Description
features_1280_h3=Locking, Lock-Timeout, Deadlocks
features_1281_th=Number of Files
features_1281_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_1282_td=test.data.db
features_1282_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_1283_td=Data file
features_1283_th=Type of Lock
features_1284_td=Contains the data for all tables
features_1284_th=SQL Statement
features_1285_td=Format\:<database>.data.db
features_1285_td=Read
features_1286_td=1 per database
features_1286_td=SELECT * FROM TEST
features_1287_td=test.index.db
features_1287_td=CALL SELECT MAX(ID) FROM TEST
features_1288_td=Index file
features_1288_td=SCRIPT
features_1289_td=Contains the data for all (btree) indexes
features_1299_td=Exists only if the database is open
features_1299_td=DROP INDEX ...
features_1300_td=Format\:<database>.lock.db
features_1300_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_1301_td=1 per database
features_1301_h2=Database File Layout
features_1302_td=test.trace.db
features_1302_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_1323_h3=Moving and Renaming Database Files
features_1323_td=Exists only if the database is open
features_1324_p=Database name and location are not stored inside the database names.
features_1324_td=Format\:<database>.lock.db
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_1325_td=1 per database
features_1326_p=As there is no platform specific data in the files, they can be moved to other operating systems without problems.
features_1326_td=test.trace.db
features_1327_h3=Backup
features_1327_td=Trace file
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_1328_td=Contains trace information
features_1329_p=To backup data while the database is running, the SQL command SCRIPT can be used.
features_1330_td=If the file is too big, it is renamed to <database>.trace.db.old
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_1331_td=1 per database
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_1332_td=test.14.15.lob.db
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_1333_td=Large object
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_1334_td=Contains the data for BLOB or CLOB
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_1336_td=1 per object
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_1337_td=test.123.temp.db
features_1338_h3=Compatibility Modes
features_1338_td=Temporary file
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 <code>jdbc\:h2\:~/test;MODE\=HSQLDB</code> or the SQL statement <code>SET MODE HSQLDB</code> . Here is the list of currently supported modes and the difference to the regular mode\:
features_1339_td=Contains a temporary blob or a large result set
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_1343_td=Hash index file
features_1344_td=MySQL
features_1344_td=Contains the data for a linear hash index
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_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_1347_h3=Moving and Renaming Database Files
features_1348_h2=Using the Trace Options
features_1348_p=Database name and location are not stored inside the database names.
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_1349_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_1350_li=Trace to System.out and/or a file
features_1350_p=As there is no platform specific data in the files, they can be moved to other operating systems without problems.
features_1351_li=Support for trace levels OFF, ERROR, INFO, and DEBUG
features_1351_h3=Backup
features_1352_li=The maximum size of the trace file can be set
features_1352_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_1353_li=The Java code generation is possible
features_1353_p=To backup data while the database is running, the SQL command SCRIPT can be used.
features_1354_li=Trace can be enabled at runtime by manually creating a file
features_1354_h2=Logging and Recovery
features_1355_h3=Trace Options
features_1355_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_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_1356_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_1357_p=The trace level can be changed at runtime by executing the SQL command <code>SET TRACE_LEVEL_SYSTEM_OUT level</code> (for System.out tracing) or <code>SET TRACE_LEVEL_FILE level</code> (for file tracing). Example\:
features_1357_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_1358_h3=Setting the Maximum Size of the Trace File
features_1358_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_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 <code>SET TRACE_MAX_FILE_SIZE maximumFileSizeInMB</code> . 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_1359_h2=Compatibility
features_1360_h3=Java Code Generation
features_1360_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_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_1361_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_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_1362_h3=Compatibility Modes
features_1363_p=Afterwards, you need to complete the file Trace.java before it can be compiled, for example with\:
features_1363_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 <code>jdbc\:h2\:~/test;MODE\=HSQLDB</code> or the SQL statement <code>SET MODE HSQLDB</code> . Here is the list of currently supported modes and the difference to the regular mode\:
features_1364_p=Also, the user name and password needs to be set, because they are not listed in the trace file.
features_1364_th=Mode
features_1365_h3=Enabling the Trace Option at Runtime by Manually Creating a File
features_1365_th=Differences
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_1366_td=PostgreSQL
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_1367_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_1368_h2=Read Only Databases
features_1368_td=MySQL
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_1369_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_1370_h2=Read Only Databases in Zip or Jar File
features_1370_td=HSQLDB
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_1371_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_1372_p=Afterwards, you can log out, and directly open the database in the zip file using the following database URL\:
features_1372_h2=Using the Trace Options
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_1373_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_1374_h2=Binary and Text Storage Formats
features_1374_li=Trace to System.out and/or a file
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_1375_li=Support for trace levels OFF, ERROR, INFO, and DEBUG
features_1376_h2=Graceful Handling of Low Disk Space Situations
features_1376_li=The maximum size of the trace file can be set
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_1377_li=The Java code generation is possible
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_1378_li=Trace can be enabled at runtime by manually creating a file
features_1379_h3=Opening a Corrupted Database
features_1379_h3=Trace Options
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_1380_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_1381_h2=Computed Columns / Function Based Index
features_1381_p=The trace level can be changed at runtime by executing the SQL command <code>SET TRACE_LEVEL_SYSTEM_OUT level</code> (for System.out tracing) or <code>SET TRACE_LEVEL_FILE level</code> (for file tracing). Example\:
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_1382_h3=Setting the Maximum Size of the Trace File
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_1383_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 <code>SET TRACE_MAX_FILE_SIZE maximumFileSizeInMB</code> . 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_1384_h2=Multi-Dimensional Indexes
features_1384_h3=Java Code Generation
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_1385_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_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_1386_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_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_1387_p=Afterwards, you need to complete the file Trace.java before it can be compiled, for example with\:
features_1388_h2=Using Passwords
features_1388_p=Also, the user name and password needs to be set, because they are not listed in the trace file.
features_1389_h3=Using Secure Passwords
features_1389_h3=Enabling the Trace Option at Runtime by Manually Creating a File
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_1390_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_1391_p=i'sE2rtPiUKtT (it's easy to remember this password if you know the trick)
features_1391_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_1392_h3=Passwords\:Using Char Arrays instead of Strings
features_1392_h2=Read Only Databases
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_1393_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_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_1394_h2=Read Only Databases in Zip or Jar 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_1395_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_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_1396_p=Afterwards, you can log out, and directly open the database in the zip file using the following database URL\:
features_1397_h3=Passing the User Name and/or Password in the URL
features_1397_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_1398_p=Instead of passing the user name as a separate parameter as in <code>Connection conn \=DriverManager. getConnection("jdbc\:h2\:~/test", "sa", "123");</code> the user name (and/or password) can be supplied in the URL itself\:<code>Connection conn \=DriverManager. getConnection("jdbc\:h2\:~/test;USER\=sa;PASSWORD\=123");</code> The settings in the URL override the settings passed as a separate parameter.
features_1398_h2=Binary and Text Storage Formats
features_1399_h2=User Defined Functions and Stored Procedures
features_1399_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_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_1400_h2=Graceful Handling of Low Disk Space Situations
features_1401_p=The Java function must be registered in the database by calling CREATE ALIAS\:
features_1401_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_1402_p=For a complete sample application, see src/test/org/h2/samples/Function.java.
features_1402_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_1403_h3=Function Data Type Mapping
features_1403_h3=Opening a Corrupted Database
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_1404_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_1405_h3=Functions that require a Connection
features_1405_h2=Computed Columns / Function Based Index
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_1406_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_1407_h3=Functions throwing an Exception
features_1407_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_1408_p=If a function throws an Exception, then the current statement is rolled back and the exception is thrown to the application.
features_1408_h2=Multi-Dimensional Indexes
features_1409_h3=Functions returning a Result Set
features_1409_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_1410_p=Functions may returns a result set. Such a function can be called with the CALL statement\:
features_1410_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_1411_h3=Using SimpleResultSet
features_1411_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_1412_p=A function that returns a result set can create this result set from scratch using the SimpleResultSet tool\:
features_1412_h2=Using Passwords
features_1413_h3=Using a Function as a Table
features_1413_h3=Using Secure Passwords
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_1414_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_1415_h2=Triggers
features_1415_p=i'sE2rtPiUKtT (it's easy to remember this password if you know the trick)
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_1416_h3=Passwords\:Using Char Arrays instead of Strings
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_1417_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_1418_p=The trigger can be used to veto a change, by throwing a SQL Exception.
features_1418_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_1419_h2=Compacting a Database
features_1419_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_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_1420_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_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_1421_h3=Passing the User Name and/or Password in the URL
features_1422_h2=Cache Settings
features_1422_p=Instead of passing the user name as a separate parameter as in <code>Connection conn \=DriverManager. getConnection("jdbc\:h2\:~/test", "sa", "123");</code> the user name (and/or password) can be supplied in the URL itself\:<code>Connection conn \=DriverManager. getConnection("jdbc\:h2\:~/test;USER\=sa;PASSWORD\=123");</code> The settings in the URL override the settings passed as a separate parameter.
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_1423_h2=User Defined Functions and Stored Procedures
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_1424_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_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_1425_p=The Java function must be registered in the database by calling CREATE ALIAS\:
features_1426_h2=Why Java
features_1426_p=For a complete sample application, see src/test/org/h2/samples/Function.java.
features_1427_p=A few reasons using a Java database are\:
features_1427_h3=Function Data Type Mapping
features_1428_li=Very simple to integrate in Java applications
features_1428_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_1429_li=Support for many different platforms
features_1429_h3=Functions that require a Connection
features_1430_li=More secure than native applications (no buffer overflows)
features_1430_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_1431_li=User defined functions (or triggers) run very fast
features_1431_h3=Functions throwing an Exception
features_1432_li=Unicode support
features_1432_p=If a function throws an Exception, then the current statement is rolled back and the exception is thrown to the application.
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_1433_h3=Functions returning a Result Set
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_1434_p=Functions may returns a result set. Such a function can be called with the CALL statement\:
features_1435_p=Java is also future proof\:A lot of companies support Java, and it is now open source.
features_1435_h3=Using SimpleResultSet
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.
features_1436_p=A function that returns a result set can create this result set from scratch using the SimpleResultSet tool\:
features_1437_h3=Using a Function as a Table
features_1438_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_1439_h2=Triggers
features_1440_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_1441_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_1442_p=The trigger can be used to veto a change, by throwing a SQL Exception.
features_1443_h2=Compacting a Database
features_1444_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_1445_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_1446_h2=Cache Settings
features_1447_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_1448_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_1449_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_1450_h2=Why Java
features_1451_p=A few reasons using a Java database are\:
features_1452_li=Very simple to integrate in Java applications
features_1453_li=Support for many different platforms
features_1454_li=More secure than native applications (no buffer overflows)
features_1455_li=User defined functions (or triggers) run very fast
features_1456_li=Unicode support
features_1457_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_1458_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_1459_p=Java is also future proof\:A lot of companies support Java, and it is now open source.
features_1460_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 <a href\="search.html">index</a> .
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 <a href\="search.html">index</a> .
tutorial_1134_p=H2 supports Lucene full text search and native full text search implementation.
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_1135_h3=Using the Native Full Text Search
tutorial_1136_p=To initialize, call\:
tutorial_1136_p=To initialize, call\:
tutorial_1137_p=Afterwards, you can create a full text index for a table using\:
tutorial_1137_p=You need to initialize it in each database where you want to use it. 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_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_1139_p=You can also call the index from within a Java application\:
tutorial_1140_h3=Using the Lucene Fulltext Search
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_1141_p=To use the Lucene full text search, you need the Lucene library in the classpath. How his is done depends on the application; if you use the H2 Console, you can add the Lucene jar file to the the environment variables H2DRIVERS or CLASSPATH. To initialize the Lucene full text search in a database, call\:
tutorial_1142_p=Afterwards, you can create a full text index for a table using\:
tutorial_1142_p=You need to initialize it in each database where you want to use it. 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_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\:
tutorial_1144_p=You can also call the index from within a Java application\:
