Skip to content

H
h2database
切换分支/标签
查找文件
Normal viewHistoryPermalink
faq.html 8.6 KB
Newer Older
Thomas Mueller's avatar
use unix newline, no trailing spaces
1e7406b5
 
Thomas Mueller committed
1 2 
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<!--
Thomas Mueller's avatar
update copyright
84595889
 
Thomas Mueller committed
3 
Copyright 2004-2009 H2 Group. Multiple-Licensed under the H2 License, Version 1.0,
Thomas Mueller's avatar
use unix newline, no trailing spaces
1e7406b5
 
Thomas Mueller committed
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 
and under the Eclipse Public License, Version 1.0
(http://h2database.com/html/license.html).
Initial Developer: H2 Group
-->
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
<head><meta http-equiv="Content-Type" content="text/html;charset=utf-8" /><title>
Frequently Asked Questions
</title><link rel="stylesheet" type="text/css" href="stylesheet.css" />
<!-- [search] { -->
<script type="text/javascript" src="navigation.js"></script>
</head><body onload="frameMe();">
<table class="content"><tr class="content"><td class="content"><div class="contentDiv">
<!-- } -->

<h1>Frequently Asked Questions</h1>

<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="#query_slow">
    My Query is Slow</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="#gcj">
    Is the GCJ Version Stable? Faster?</a><br />
<a href="#translate">
    How to Translate this Project?</a><br />

<br /><a name="known_bugs"></a>
<h3>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>
Thomas Mueller's avatar
--no commit message
42967945
 
Thomas Mueller committed
49 
<ul><li>Tomcat and Glassfish 3 set most static fields (final or non-final) to null when
Thomas Mueller's avatar
--no commit message
8000e8e9
 
Thomas Mueller committed
50 51 52 
    unloading a web application. This can cause a NullPointerException 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
Thomas Mueller's avatar
Tomcat sets fields to null when unloading the web application.
cde29719
 
Thomas Mueller committed
53 
    system property org.apache.catalina.loader.WebappClassLoader.ENABLE_CLEAR_REFERENCES
Thomas Mueller's avatar
--no commit message
8000e8e9
 
Thomas Mueller committed
54 
    to false, however Tomcat may then run out of memory. A known workaround is to
Thomas Mueller's avatar
--no commit message
42967945
 
Thomas Mueller committed
55 
    put the h2.jar file in a shared <code>lib</code> directory (common/lib).
Thomas Mueller's avatar
--no commit message
8000e8e9
 
Thomas Mueller committed
56 57 
</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
Thomas Mueller's avatar
Tomcat sets fields to null when unloading the web application.
cde29719
 
Thomas Mueller committed
58 
    when used in combination with other joins.
Thomas Mueller's avatar
use unix newline, no trailing spaces
1e7406b5
 
Thomas Mueller committed
59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 
</li><li>When using Install4j before 4.1.4 on Linux and enabling 'pack200',
    the h2*.jar becomes corrupted by the install process, causing application failure.
    A workaround is to add an empty file h2*.jar.nopack next to the h2*.jar file.
    This problem is solved in Install4j 4.1.4.
</li></ul>

<br /><a name="open_source"></a>
<h3>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>

<br /><a name="query_slow"></a>
<h3>My Query is Slow</h3>
<p>
Slow SELECT (or DELETE, UPDATE, MERGE) statement can have multiple reasons.
Follow this checklist:
</p>
<ul>
Thomas Mueller's avatar
--no commit message
502d8756
 
Thomas Mueller committed
79 
<li>Run ANALYZE (see documentation for details).
Thomas Mueller's avatar
use unix newline, no trailing spaces
1e7406b5
 
Thomas Mueller committed
80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 
</li><li>Run the query with EXPLAIN and check if indexes are used (see documentation for details).
</li><li>If required, create additional indexes and try again using ANALYZE and EXPLAIN.
</li><li>If it doesn't help please report the problem.
</li>
</ul>

<br /><a name="create_database"></a>
<h3>How to Create a New Database?</h3>
<p>
By default, a new database is automatically created if it does not yet exist.
</p>

<br /><a name="connect"></a>
<h3>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>

<br /><a name="database_files"></a>
<h3>Where are the Database Files Stored?</h3>
<p>
When using database URLs like jdbc:h2:~/test, the database is stored in the user directory.
Thomas Mueller's avatar
Improve documentation
df90fb37
 
Thomas Mueller committed
108 
For Windows, this is usually "C:\Documents and Settings\&lt;userName&gt;".
Thomas Mueller's avatar
use unix newline, no trailing spaces
1e7406b5
 
Thomas Mueller committed
109 
If the base directory is not set (as in jdbc:h2:test), the database files are stored in the directory where the application is started
Thomas Mueller's avatar
Improve documentation
df90fb37
 
Thomas Mueller committed
110 
(the current working directory). When using the H2 Console application from the start menu, this is "&lt;Installation Directory&gt;/bin".
Thomas Mueller's avatar
use unix newline, no trailing spaces
1e7406b5
 
Thomas Mueller committed
111 
The base directory can be set in the database URL. A fixed or relative path can be used. When using the URL
Thomas Mueller's avatar
Improve documentation
df90fb37
 
Thomas Mueller committed
112 113 
jdbc:h2:file:data/sample, the database is stored in the directory "data" (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).
Thomas Mueller's avatar
use unix newline, no trailing spaces
1e7406b5
 
Thomas Mueller committed
114 115 116 117 118 119 
Example: jdbc:h2:file:C:/data/test
</p>

<br /><a name="size_limit"></a>
<h3>What is the Size Limit (Maximum Size) of a Database?</h3>
<p>
Thomas Mueller's avatar
Improved docs
0e440644
 
Thomas Mueller committed
120 
See <a href="advanced.html#limits_limitations">Limits and Limitations</a>.
Thomas Mueller's avatar
use unix newline, no trailing spaces
1e7406b5
 
Thomas Mueller committed
121 122 123 124 125 126 127 
</p>

<br /><a name="reliable"></a>
<h3>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
Thomas Mueller's avatar
Improve documentation
df90fb37
 
Thomas Mueller committed
128 
are run regularly. But there are probably still
buckyballs's avatar
Documentation: experimental features and special causes of OutOfMemoryError. …
6225433e
 
buckyballs committed
129 
bugs that have not yet been found (as with most software). Some features are known
Thomas Mueller's avatar
Improve documentation
df90fb37
 
Thomas Mueller committed
130 131 
to be dangerous, they are only supported for situations where performance is more important
than reliability. Those dangerous features are:
Thomas Mueller's avatar
use unix newline, no trailing spaces
1e7406b5
 
Thomas Mueller committed
132 133 
</p>
<ul>
Thomas Mueller's avatar
Improve documentation
df90fb37
 
Thomas Mueller committed
134 
<li>Disabling the transaction log mechanism using SET LOG 0.
Thomas Mueller's avatar
use unix newline, no trailing spaces
1e7406b5
 
Thomas Mueller committed
135 
</li><li>Using the transaction isolation level READ_UNCOMMITTED (LOCK_MODE 0) while at the same time using multiple
Thomas Mueller's avatar
Improve documentation
df90fb37
 
Thomas Mueller committed
136 137 138 
    connections.
</li><li>Disabling database file protection using FILE_LOCK=NO in the database URL.
</li><li>Disabling referential integrity using SET REFERENTIAL_INTEGRITY FALSE.
Thomas Mueller's avatar
use unix newline, no trailing spaces
1e7406b5
 
Thomas Mueller committed
139 140 141 
</li></ul>
<p>
In addition to that, running out of memory should be avoided.
Thomas Mueller's avatar
Improve documentation
df90fb37
 
Thomas Mueller committed
142 
In older versions, OutOfMemory errors while using the database could corrupt a databases.
Thomas Mueller's avatar
use unix newline, no trailing spaces
1e7406b5
 
Thomas Mueller committed
143 144 145 146 147 
</p>
<p>
Areas that are not fully tested:
</p>
<ul>
Thomas Mueller's avatar
prepare release
e7197fcc
 
Thomas Mueller committed
148 
<li>Platforms other than Windows XP, Linux, Mac OS X, or JVMs other than Sun 1.5 or 1.6
Thomas Mueller's avatar
use unix newline, no trailing spaces
1e7406b5
 
Thomas Mueller committed
149 150 151 152 153 154 
</li><li>The features AUTO_SERVER and AUTO_RECONNECT
</li><li>The MVCC (multi version concurrency) mode
</li><li>Cluster mode, 2-phase commit, savepoints
</li><li>24/7 operation
</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
buckyballs's avatar
Documentation: experimental features and special causes of OutOfMemoryError. …
6225433e
 
buckyballs committed
155 156 
</li><li>Fulltext search
</li><li>Operations on LOBs over 2 GB
Thomas Mueller's avatar
use unix newline, no trailing spaces
1e7406b5
 
Thomas Mueller committed
157 158 159 160 161 162 163 
</li></ul>

<p>
Areas considered Experimental:
</p>
<ul>
<li>The PostgreSQL server
buckyballs's avatar
Documentation: experimental features and special causes of OutOfMemoryError. …
6225433e
 
buckyballs committed
164 165 
</li><li>The new page store
</li><li>Multi-threading within the engine using SET MULTI_THREADED=1
Thomas Mueller's avatar
use unix newline, no trailing spaces
1e7406b5
 
Thomas Mueller committed
166 167 168 169 170 171 172 173 174 175 176 
</li><li>Compatibility modes for other databases (only some features are implemented)
</li></ul>

<br /><a name="slow_open"></a>
<h3>Why is Opening my Database Slow?</h3>
<p>
If it takes a long time to open a database, in most cases it was not closed the last time.
This is specially a problem for larger databases.
To close a database, close all connections to it before the application ends, or execute
the command SHUTDOWN. The database is also closed when the virtual machine exits normally
by using a shutdown hook. However killing a Java process or calling Runtime.halt will prevent this.
Thomas Mueller's avatar
Improve documentation
df90fb37
 
Thomas Mueller committed
177 178 
The reason why opening is slow in this situations is that indexes are re-created.
If you  can not guarantee the database is closed, consider using SET LOG 2 (see SQL Grammar).
Thomas Mueller's avatar
use unix newline, no trailing spaces
1e7406b5
 
Thomas Mueller committed
179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 
</p>
<p>
To find out what the problem is, open the database in embedded mode using the H2 Console.
This will print progress information. If you have many 'Creating index' lines it is an indication that the
database was not closed the last time.
</p>
<p>
Other possible reasons are: the database is very big (many GB), or contains linked tables
that are slow to open.
</p>

<br /><a name="gcj"></a>
<h3>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>

<br /><a name="translate"></a>
<h3>How to Translate this Project?</h3>
<p>
For more information, see
<a href="build.html#translating">Build/Translating</a>.
</p>

<!-- [close] { --></div></td></tr></table><!-- } --><!-- analytics --></body></html>