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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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Frequently Asked Questions
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<h1>Frequently Asked Questions</h1>
<a href="#support">
I Have a Problem or Feature Request</a><br />
<a href="#known_bugs">
Are there Known Bugs? When is the Next Release?</a><br />
<a href="#open_source">
Is this Database Engine Open Source?</a><br />
<a href="#create_database">
How to Create a New Database?</a><br />
<a href="#connect">
How to Connect to a Database?</a><br />
<a href="#database_files">
Where are the Database Files Stored?</a><br />
<a href="#size_limit">
What is the Size Limit (Maximum Size) of a Database?</a><br />
<a href="#reliable">
Is it Reliable?</a><br />
<a href="#slow_open">
Why is Opening my Database Slow?</a><br />
<a href="#query_slow">
My Query is Slow</a><br />
<a href="#database_slow">
H2 is Very Slow</a><br />
<a href="#column_names_incorrect">
Column Names are Incorrect?</a><br />
<a href="#float_is_double">
Float is Double?</a><br />
<a href="#gcj">
Is the GCJ Version Stable? Faster?</a><br />
<a href="#how_to_translate">
How to Translate this Project?</a><br />
<a href="#how_to_contribute">
How to Contribute to this Project?</a><br />
<h3 id="support">I Have a Problem or Feature Request</h3>
<p>
Please read the <a href="build.html#support">support checklist</a>.
</p>
<h3 id="known_bugs">Are there Known Bugs? When is the Next Release?</h3>
<p>
Usually, bugs get fixes as they are found. There is a release every few weeks.
Here is the list of known and confirmed issues:
</p>
<ul><li>Apache Harmony: there seems to be a bug in Harmony that affects H2.
See <a href="https://issues.apache.org/jira/browse/HARMONY-6505">HARMONY-6505</a>.
</li><li>Tomcat and Glassfish 3 set most static fields (final or non-final) to <code>null</code> when
unloading a web application. This can cause a <code>NullPointerException</code> in H2 versions
1.1.107 and older, and may still not work in newer versions. Please report it if you
run into this issue. In Tomcat >= 6.0 this behavior can be disabled by setting the
system property <code>org.apache.catalina.loader.WebappClassLoader.ENABLE_CLEAR_REFERENCES=false</code>,
however Tomcat may then run out of memory. A known workaround is to
put the <code>h2*.jar</code> file in a shared <code>lib</code> directory
(<code>common/lib</code>).
</li><li>Some problems have been found with right outer join. Internally, it is converted
to left outer join, which does not always produce the same results as other databases
when used in combination with other joins.
</li><li>When using Install4j before 4.1.4 on Linux and enabling <code>pack200</code>,
the <code>h2*.jar</code> becomes corrupted by the install process, causing application failure.
A workaround is to add an empty file <code>h2*.jar.nopack</code>
next to the <code>h2*.jar</code> file.
This problem is solved in Install4j 4.1.4.
</li></ul>
<p>
For a complete list, see <a href="http://code.google.com/p/h2database/issues/list">Open Issues</a>.
</p>
<h3 id="open_source">Is this Database Engine Open Source?</h3>
<p>
Yes. It is free to use and distribute, and the source code is included.
See also under license.
</p>
<h3 id="create_database">How to Create a New Database?</h3>
<p>
By default, a new database is automatically created if it does not yet exist.
See <a href="tutorial.html#creating_new_databases">Creating New Databases</a>.
</p>
<h3 id="connect">How to Connect to a Database?</h3>
<p>
The database driver is <code>org.h2.Driver</code>,
and the database URL starts with <code>jdbc:h2:</code>.
To connect to a database using JDBC, use the following code:
</p>
<pre>
Class.forName("org.h2.Driver");
Connection conn = DriverManager.getConnection("jdbc:h2:~/test", "sa", "");
</pre>
<h3 id="database_files">Where are the Database Files Stored?</h3>
<p>
When using database URLs like <code>jdbc:h2:~/test</code>,
the database is stored in the user directory.
For Windows, this is usually <code>C:\Documents and Settings\<userName></code>.
If the base directory is not set (as in <code>jdbc:h2:test</code>),
the database files are stored in the directory where the application is started
(the current working directory). When using the H2 Console application from the start menu,
this is <code><Installation Directory>/bin</code>.
The base directory can be set in the database URL. A fixed or relative path can be used. When using the URL
<code>jdbc:h2:file:data/sample</code>, the database is stored in the directory
<code>data</code> (relative to the current working directory).
The directory is created automatically if it does not yet exist. It is also possible to use the
fully qualified directory name (and for Windows, drive name).
Example: <code>jdbc:h2:file:C:/data/test</code>
</p>
<h3 id="size_limit">What is the Size Limit (Maximum Size) of a Database?</h3>
<p>
See <a href="advanced.html#limits_limitations">Limits and Limitations</a>.
</p>
<h3 id="reliable">Is it Reliable?</h3>
<p>
That is not easy to say. It is still a quite new product. A lot of tests have been written,
and the code coverage of these tests is very high. Randomized stress tests
are run regularly. But there are probably still
bugs that have not yet been found (as with most software). Some features are known
to be dangerous, they are only supported for situations where performance is more important
than reliability. Those dangerous features are:
</p>
<ul><li>Disabling the transaction log or FileDescriptor.sync() using LOG=0 or LOG=1.
</li><li>Using the transaction isolation level <code>READ_UNCOMMITTED</code>
(<code>LOCK_MODE 0</code>) while at the same time using multiple
connections.
</li><li>Disabling database file protection using (setting <code>FILE_LOCK</code> to
<code>NO</code> in the database URL).
</li><li>Disabling referential integrity using <code>SET REFERENTIAL_INTEGRITY FALSE</code>.
</li></ul>
<p>
In addition to that, running out of memory should be avoided.
In older versions, OutOfMemory errors while using the database could corrupt a databases.
</p>
<p>
This database is well tested using automated test cases. The tests run every night
and run for more than one hour. But not all areas of this database are equally well tested.
When using one of the following features for production, please ensure your use case
is well tested (if possible with automated test cases). The areas that are not well tested are:
</p>
<ul>
<li>Platforms other than Windows XP, Linux, Mac OS X, or JVMs other than Sun 1.5 or 1.6
</li><li>The features <code>AUTO_SERVER</code> and <code>AUTO_RECONNECT</code>
</li><li>The file locking method 'Serialized'
</li><li>Cluster mode, 2-phase commit, savepoints
</li><li>24/7 operation
</li><li>Fulltext search
</li><li>Operations on LOBs over 2 GB
</li><li>Some operations on databases larger than 500 MB may be slower than expected
</li><li>The optimizer may not always select the best plan
</li></ul>
<p>
Areas considered experimental are:
</p>
<ul>
<li>The PostgreSQL server
</li><li>Multi-threading within the engine using <code>SET MULTI_THREADED=1</code>
</li><li>Compatibility modes for other databases (only some features are implemented)
</li><li>The soft reference cache (<code>CACHE_TYPE=SOFT_LRU</code>). It might not improve performance,
and out of memory issues have been reported.
</li></ul>
<p>
Some users have reported that after a power failure, the database cannot be opened sometimes.
In this case, use a backup of the database or the Recover tool. Please report such problems.
The plan is that the database automatically recovers in all situations.
</p>
<h3 id="slow_open">Why is Opening my Database Slow?</h3>
<p>
To find out what the problem is, use the H2 Console and click on "Test Connection"
instead of "Login". After the "Login Successful" appears, click on it (it's a link).
This will list the top stack traces. Then either analyze this yourself, or
post those stack traces in the Google Group.
</p>
<p>
Other possible reasons are: the database is very big (many GB), or contains linked tables
that are slow to open.
</p>
<h3 id="query_slow">My Query is Slow</h3>
<p>
Slow <code>SELECT</code> (or <code>DELETE, UPDATE, MERGE</code>)
statement can have multiple reasons. Follow this checklist:
</p>
<ul>
<li>Run <code>ANALYZE</code> (see documentation for details).
</li><li>Run the query with <code>EXPLAIN</code> and check if indexes are used
(see documentation for details).
</li><li>If required, create additional indexes and try again using
<code>ANALYZE</code> and <code>EXPLAIN</code>.
</li><li>If it doesn't help please report the problem.
</li>
</ul>
<h3 id="database_slow">H2 is Very Slow</h3>
<p>
By default, H2 closes the database when the last connection is closed.
If your application closes the only connection after each operation,
the database is opened and closed a lot, which is quite slow.
There are multiple ways to solve this problem, see
<a href="performance.html#database_performance_tuning">Database Performance Tuning</a>.
</p>
<h3 id="column_names_incorrect">Column Names are Incorrect?</h3>
<p>
For the query <code>SELECT ID AS X FROM TEST</code> the method
<code>ResultSetMetaData.getColumnName()</code> returns <code>ID</code>, I expect it to
return <code>X</code>. What's wrong?
</p>
<p>
This is not a bug. According the the JDBC specification, the method
<code>ResultSetMetaData.getColumnName()</code> should return the name of the column
and not the alias name. If you need the alias name, use
<a href="http://java.sun.com/javase/6/docs/api/java/sql/ResultSetMetaData.html#getColumnLabel(int)"><code>ResultSetMetaData.getColumnLabel()</code></a>.
Some other database don't work like this yet (they don't follow the JDBC specification).
If you need compatibility with those databases, use the <a href="features.html#compatibility">Compatibility Mode</a>,
or append <a href="../javadoc/org/h2/constant/DbSettings.html#ALIAS_COLUMN_NAME"><code>;ALIAS_COLUMN_NAME=TRUE</code></a> to the database URL.
</p>
<p>
This also applies to DatabaseMetaData calls that return a result set.
The columns in the JDBC API are column labels, not column names.
</p>
<h3 id="float_is_double">Float is Double?</h3>
<p>
For a table defined as <code>CREATE TABLE TEST(X FLOAT)</code> the method
<code>ResultSet.getObject()</code> returns a <code>java.lang.Double</code>, I expect it to
return a <code>java.lang.Float</code>. What's wrong?
</p>
<p>
This is not a bug. According the the JDBC specification, the JDBC data type <code>FLOAT</code>
is equivalent to <code>DOUBLE</code>, and both are mapped to <code>java.lang.Double</code>.
See also
<a href="http://java.sun.com/j2se/1.5.0/docs/guide/jdbc/getstart/mapping.html#1055162">
Mapping SQL and Java Types - 8.3.10 FLOAT</a>.</p>
<h3 id="gcj">Is the GCJ Version Stable? Faster?</h3>
<p>
The GCJ version is not as stable as the Java version.
When running the regression test with the GCJ version, sometimes the application just stops
at what seems to be a random point without error message.
Currently, the GCJ version is also slower than when using the Sun VM.
However, the startup of the GCJ version is faster than when using a VM.
</p>
<h3 id="how_to_translate">How to Translate this Project?</h3>
<p>
For more information, see
<a href="build.html#translating">Build/Translating</a>.
</p>
<h3 id="how_to_contribute">How to Contribute to this Project?</h3>
<p>
There are various way to help develop an open source project like H2. The first step could be to
<a href="build.html#translating">translate</a> the error messages and the GUI to your native language.
Then, you could
<a href="build.html#providing_patches">provide patches</a>.
Please start with small patches. That could be adding a test case to improve the
<a href="build.html#automated">code coverage</a> (the target code coverage for this project is 90%, higher is better).
You will have to <a href="build.html">develop, build and run the tests</a>.
Once you are familiar with the code, you could implement missing features from the
<a href="roadmap.html">feature request list</a>.
I suggest to start with very small features that are easy to implement.
Keep in mind to provide test cases as well.
</p>
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