基于Java实现的 H2 数据库引擎 1.3.161 发布
jopen 13年前
<p>H2是一个短小精干的嵌入式数据库引擎，主要的特性包括：</p> <ul> <li>免费、开源、快速</li> <li>嵌入式的数据库服务器，支持集群</li> <li>提供JDBC、ODBC访问接口，提供基于浏览器的控制台管理程序</li> <li>Java编写，可使用GCJ和IKVM.NET编译</li> <li>短小精干的软件，1M左右。</li> </ul> <p>该版本改进了对 Java 7 文件系统抽象层的兼容性，大数据库的处理速度更快了。<br /> <img title="h2-logo.png" border="0" alt="h2-logo.png" src="https://simg.open-open.com/show/50bc808753c2dd8f66ab60b2796f5a6b.png" width="136" height="74" /></p> <table class="ke-zeroborder"> <tbody> <tr> <td style="border-bottom:0px;border-left:0px;background-color:#eee;border-top:0px;border-right:0px;" colspan="2"><span style="font-weight:bold;">下载：</span><br /> Version 1.3.161 (2011-10-28) </td> </tr> <tr> <td style="border-bottom:0px;border-left:0px;background-color:#eee;border-top:0px;border-right:0px;"><a href="/misc/goto?guid=4958196663549552520"><img style="border-bottom:#00f 1px solid;border-left:#00f 1px solid;border-top:#00f 1px solid;border-right:#00f 1px solid;" alt="Download this database" src="https://simg.open-open.com/show/93e0e3a9e442a6367228d523ac9685bd.png" width="22" height="20" /></a> </td> <td style="border-bottom:0px;border-left:0px;background-color:#eee;vertical-align:middle;border-top:0px;border-right:0px;"><a href="/misc/goto?guid=4958196663549552520">Windows Installer (4 MB)</a> </td> </tr> <tr> <td style="border-bottom:0px;border-left:0px;background-color:#eee;border-top:0px;border-right:0px;"><a href="/misc/goto?guid=4958196664977088441"><img style="border-bottom:#00f 1px solid;border-left:#00f 1px solid;border-top:#00f 1px solid;border-right:#00f 1px solid;" alt="Download this database" src="https://simg.open-open.com/show/93e0e3a9e442a6367228d523ac9685bd.png" width="22" height="20" /></a> </td> <td style="border-bottom:0px;border-left:0px;background-color:#eee;vertical-align:middle;border-top:0px;border-right:0px;"><a href="/misc/goto?guid=4958196664977088441">All Platforms (zip, 5 MB)</a> </td> </tr> <tr> <td style="border-bottom:0px;border-left:0px;background-color:#eee;border-top:0px;border-right:0px;" colspan="2"><a href="/misc/goto?guid=4958196666374701101">All Downloads</a></td> </tr> </tbody> </table> <p> </p> <h2 id="feature_list">Feature List</h2> <h3>Main Features</h3> <ul> <li>Very fast database engine </li> <li>Open source </li> <li>Written in Java </li> <li>Supports standard SQL, JDBC API </li> <li>Embedded and Server mode, Clustering support </li> <li>Strong security features </li> <li>The PostgreSQL ODBC driver can be used </li> <li>Multi version concurrency </li> </ul> <h3>Additional Features</h3> <ul> <li>Disk based or in-memory databases and tables, read-only database support, temporary tables </li> <li>Transaction support (read committed and serializable transaction isolation), 2-phase-commit </li> <li>Multiple connections, table level locking </li> <li>Cost based optimizer, using a genetic algorithm for complex queries, zero-administration </li> <li>Scrollable and updatable result set support, large result set, external result sorting, functions can return a result set </li> <li>Encrypted database (AES or XTEA), SHA-256 password encryption, encryption functions, SSL </li> </ul> <h3>SQL Support</h3> <ul> <li>Support for multiple schemas, information schema </li> <li>Referential integrity / foreign key constraints with cascade, check constraints </li> <li>Inner and outer joins, subqueries, read only views and inline views </li> <li>Triggers and Java functions / stored procedures </li> <li>Many built-in functions, including XML and lossless data compression </li> <li>Wide range of data types including large objects (BLOB/CLOB) and arrays </li> <li>Sequence and autoincrement columns, computed columns (can be used for function based indexes) </li> <li><code class="notranslate">ORDER BY, GROUP BY, HAVING, UNION, LIMIT, TOP</code> </li> <li>Collation support, including support for the ICU4J library </li> <li>Support for users and roles </li> <li>Compatibility modes for IBM DB2, Apache Derby, HSQLDB, MS SQL Server, MySQL, Oracle, and PostgreSQL. </li> </ul> <h3>Security Features</h3> <ul> <li>Includes a solution for the SQL injection problem </li> <li>User password authentication uses SHA-256 and salt </li> <li>For server mode connections, user passwords are never transmitted in plain text over the network (even when using insecure connections; this only applies to the TCP server and not to the H2 Console however; it also doesn't apply if you set the password in the database URL) </li> <li>All database files (including script files that can be used to backup data) can be encrypted using AES-128 and XTEA encryption algorithms </li> <li>The remote JDBC driver supports TCP/IP connections over SSL/TLS </li> <li>The built-in web server supports connections over SSL/TLS </li> <li>Passwords can be sent to the database using char arrays instead of Strings </li> </ul> <h3>Other Features and Tools</h3> <ul> <li>Small footprint (smaller than 1 MB), low memory requirements </li> <li>Multiple index types (b-tree, tree, hash) </li> <li>Support for multi-dimensional indexes </li> <li>CSV (comma separated values) file support </li> <li>Support for linked tables, and a built-in virtual 'range' table </li> <li>Supports the <code class="notranslate">EXPLAIN PLAN</code> statement; sophisticated trace options </li> <li>Database closing can be delayed or disabled to improve the performance </li> <li>Web-based Console application (translated to many languages) with autocomplete </li> <li>The database can generate SQL script files </li> <li>Contains a recovery tool that can dump the contents of the database </li> <li>Support for variables (for example to calculate running totals) </li> <li>Automatic re-compilation of prepared statements </li> <li>Uses a small number of database files </li> <li>Uses a checksum for each record and log entry for data integrity </li> <li>Well tested (high code coverage, randomized stress tests) </li> </ul> <h2 id="comparison">Comparison to Other Database Engines</h2> <p>This comparison is based on H2 1.3, <a href="/misc/goto?guid=4958196667109818032">Apache Derby version 10.8</a>, <a href="/misc/goto?guid=4958195073669077276">HSQLDB 2.2</a>, <a href="/misc/goto?guid=4958196668509493139">MySQL 5.5</a>, <a href="/misc/goto?guid=4958183761065211007">PostgreSQL 9.0</a>. </p> <table class="main ke-zeroborder"> <tbody> <tr> <th>Feature</th> <th>H2</th> <th>Derby</th> <th>HSQLDB</th> <th>MySQL</th> <th>PostgreSQL</th> </tr> <tr> <td>Pure Java</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareN">No</td> <td class="compareN">No</td> </tr> <tr> <td>Embedded Mode (Java)</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareN">No</td> <td class="compareN">No</td> </tr> <tr> <td>In-Memory Mode</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareN">No</td> <td class="compareN">No</td> </tr> <tr></tr> <tr> <td>Explain Plan</td> <td class="compareY">Yes</td> <td class="compareY">Yes *12</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> </tr> <tr> <td>Built-in Clustering / Replication</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareN">No</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> </tr> <tr> <td>Encrypted Database</td> <td class="compareY">Yes</td> <td class="compareY">Yes *10</td> <td class="compareY">Yes *10</td> <td class="compareN">No</td> <td class="compareN">No</td> </tr> <tr> <td>Linked Tables</td> <td class="compareY">Yes</td> <td class="compareN">No</td> <td class="compareY">Partially *1</td> <td class="compareY">Partially *2</td> <td class="compareN">No</td> </tr> <tr> <td>ODBC Driver</td> <td class="compareY">Yes</td> <td class="compareN">No</td> <td class="compareN">No</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> </tr> <tr> <td>Fulltext Search</td> <td class="compareY">Yes</td> <td class="compareN">No</td> <td class="compareN">No</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> </tr> <tr> <td>Domains (User-Defined Types)</td> <td class="compareY">Yes</td> <td class="compareN">No</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> </tr> <tr> <td>Files per Database</td> <td class="compareY">Few</td> <td class="compareN">Many</td> <td class="compareY">Few</td> <td class="compareN">Many</td> <td class="compareN">Many</td> </tr> <tr> <td>Row Level Locking</td> <td class="compareY">Yes *9</td> <td class="compareY">Yes</td> <td class="compareY">Yes *9</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> </tr> <tr> <td>Multi Version Concurrency</td> <td class="compareY">Yes</td> <td class="compareN">No</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> </tr> <tr> <td>Multi-Threaded Statement Processing</td> <td class="compareN">No *11</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> </tr> <tr> <td>Role Based Security</td> <td class="compareY">Yes</td> <td class="compareY">Yes *3</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> </tr> <tr> <td>Updatable Result Sets</td> <td class="compareY">Yes</td> <td class="compareY">Yes *7</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> </tr> <tr> <td>Sequences</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareN">No</td> <td class="compareY">Yes</td> </tr> <tr> <td>Limit and Offset</td> <td class="compareY">Yes</td> <td class="compareY">Yes *13</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> </tr> <tr> <td>Window Functions</td> <td class="compareN">No *15</td> <td class="compareN">No *15</td> <td class="compareN">No</td> <td class="compareY">Yes</td> <td class="compareN">No</td> </tr> <tr> <td>Temporary Tables</td> <td class="compareY">Yes</td> <td class="compareY">Yes *4</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> </tr> <tr> <td>Information Schema</td> <td class="compareY">Yes</td> <td class="compareN">No *8</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> </tr> <tr> <td>Computed Columns</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareN">No</td> <td class="compareY">Yes *6</td> </tr> <tr> <td>Case Insensitive Columns</td> <td class="compareY">Yes</td> <td class="compareY">Yes *14</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareY">Yes *6</td> </tr> <tr> <td>Custom Aggregate Functions</td> <td class="compareY">Yes</td> <td class="compareN">No</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> <td class="compareY">Yes</td> </tr> <tr> <td>CLOB/BLOB Compression</td> <td class="compareY">Yes</td> <td class="compareN">No</td> <td class="compareN">No</td> <td class="compareN">No</td> <td class="compareY">Yes</td> </tr> <tr> <td>Footprint (jar/dll size)</td> <td>~1 MB *5</td> <td>~2 MB</td> <td>~1 MB</td> <td>~4 MB</td> <td>~6 MB</td> </tr> </tbody> </table> <p>*1 HSQLDB supports text tables.<br /> *2 MySQL supports linked MySQL tables under the name 'federated tables'.<br /> *3 Derby support for roles based security and password checking as an option.<br /> *4 Derby only supports global temporary tables.<br /> *5 The default H2 jar file contains debug information, jar files for other databases do not.<br /> *6 PostgreSQL supports functional indexes.<br /> *7 Derby only supports updatable result sets if the query is not sorted.<br /> *8 Derby doesn't support standard compliant information schema tables.<br /> *9 When using MVCC (multi version concurrency).<br /> *10 Derby and HSQLDB <a href="/misc/goto?guid=4958196669921355564">don't hide data patterns well</a>.<br /> *11 The MULTI_THREADED option is not enabled by default, and not yet supported when using MVCC.<br /> *12 Derby doesn't support the <code class="notranslate">EXPLAIN</code> statement, but it supports runtime statistics and retrieving statement execution plans.<br /> *13 Derby doesn't support the syntax <code class="notranslate">LIMIT .. [OFFSET ..]</code>, however it supports <code class="notranslate">FETCH FIRST .. ROW[S] ONLY</code>.<br /> *14 Using collations. *15 Derby and H2 support <code class="notranslate">ROW_NUMBER() OVER()</code>. </p> <h3>DaffodilDb and One$Db</h3> <p>It looks like the development of this database has stopped. The last release was February 2006. </p> <h3>McKoi</h3> <p>It looks like the development of this database has stopped. The last release was August 2004. </p> <h2 id="products_work_with">H2 in Use</h2> <p>For a list of applications that work with or use H2, see: <a href="/misc/goto?guid=4958196670659208548">Links</a>. </p> <h2 id="connection_modes">Connection Modes</h2> <p>The following connection modes are supported: </p> <ul> <li>Embedded mode (local connections using JDBC) </li> <li>Server mode (remote connections using JDBC or ODBC over TCP/IP) </li> <li>Mixed mode (local and remote connections at the same time) </li> </ul> <h3>Embedded Mode</h3> <p>In embedded mode, an application opens a database from within the same JVM using JDBC. This is the fastest and easiest connection mode. The disadvantage is that a database may only be open in one virtual machine (and class loader) at any time. As in all modes, both persistent and in-memory databases are supported. There is no limit on the number of database open concurrently, or on the number of open connections. </p> <img alt="The database is embedded in the application" src="https://simg.open-open.com/show/7ab48f60a0e0e276880e251426acaf48.png" /> <h3>Server Mode</h3> <p>When using the server mode (sometimes called remote mode or client/server mode), an application opens a database remotely using the JDBC or ODBC API. A server needs to be started within the same or another virtual machine, or on another computer. Many applications can connect to the same database at the same time, by connecting to this server. Internally, the server process opens the database(s) in embedded mode. </p> <p>The server mode is slower than the embedded mode, because all data is transferred over TCP/IP. As in all modes, both persistent and in-memory databases are supported. There is no limit on the number of database open concurrently per server, or on the number of open connections. </p> <img alt="The database is running in a server; the application connects to the server" src="https://simg.open-open.com/show/0011b4833f16ed94c20e6c230f9fe37a.png" /> <h3>Mixed Mode</h3> <p>The mixed mode is a combination of the embedded and the server mode. The first application that connects to a database does that in embedded mode, but also starts a server so that other applications (running in different processes or virtual machines) can concurrently access the same data. The local connections are as fast as if the database is used in just the embedded mode, while the remote connections are a bit slower. </p> <p>The server can be started and stopped from within the application (using the server API), or automatically (automatic mixed mode). When using the <a href="/misc/goto?guid=4958196671386438536">automatic mixed mode</a>, all clients that want to connect to the database (no matter if it's an local or remote connection) can do so using the exact same database URL. </p> <img alt="Database, server, and application run in one JVM; an application connects" src="https://simg.open-open.com/show/dfc5fa453c0210464703818fc9e37fc2.png" /> <h2 id="database_url">Database URL Overview</h2> <p>This database supports multiple connection modes and connection settings. This is achieved using different database URLs. Settings in the URLs are not case sensitive. </p> <table class="main ke-zeroborder"> <tbody> <tr> <th>Topic</th> <th>URL Format and Examples</th> </tr> <tr> <td><a href="/misc/goto?guid=4958196672122631418">Embedded (local) connection</a></td> <td class="notranslate">jdbc:h2:[file:][<path>]<databaseName><br /> jdbc:h2:~/test<br /> jdbc:h2:file:/data/sample<br /> jdbc:h2:file:C:/data/sample (Windows only)<br /> </td> </tr> <tr> <td><a href="/misc/goto?guid=4958196672859687944">In-memory (private)</a></td> <td class="notranslate">jdbc:h2:mem:</td> </tr> <tr> <td><a href="/misc/goto?guid=4958196672859687944">In-memory (named)</a></td> <td class="notranslate">jdbc:h2:mem:<databaseName><br /> jdbc:h2:mem:test_mem </td> </tr> <tr> <td><a href="/misc/goto?guid=4958196674267333674"><br /> Server mode (remote connections) using TCP/IP</a></td> <td class="notranslate">jdbc:h2:tcp://<server>[:<port>]/[<path>]<databaseName><br /> jdbc:h2:tcp://localhost/~/test<br /> jdbc:h2:tcp://dbserv:8084/~/sample<br /> jdbc:h2:tcp://localhost/mem:test<br /> </td> </tr> <tr> <td><a href="/misc/goto?guid=4958196675002493774"><br /> Server mode (remote connections) using SSL/TLS</a></td> <td class="notranslate">jdbc:h2:ssl://<server>[:<port>]/<databaseName><br /> jdbc:h2:ssl://localhost:8085/~/sample; </td> </tr> <tr> <td><a href="/misc/goto?guid=4958196675738980781">Using encrypted files</a></td> <td class="notranslate">jdbc:h2:<url>;CIPHER=[AES|XTEA]<br /> jdbc:h2:ssl://localhost/~/test;CIPHER=AES<br /> jdbc:h2:file:~/secure;CIPHER=XTEA<br /> </td> </tr> <tr> <td><a href="/misc/goto?guid=4958196676476391179">File locking methods</a></td> <td class="notranslate">jdbc:h2:<url>;FILE_LOCK={FILE|SOCKET|NO}<br /> jdbc:h2:file:~/private;CIPHER=XTEA;FILE_LOCK=SOCKET<br /> </td> </tr> <tr> <td><a href="/misc/goto?guid=4958196677202542990">Only open if it already exists</a></td> <td class="notranslate">jdbc:h2:<url>;IFEXISTS=TRUE<br /> jdbc:h2:file:~/sample;IFEXISTS=TRUE<br /> </td> </tr> <tr> <td><a href="/misc/goto?guid=4958196677945751515">Don't close the database when the VM exits</a></td> <td class="notranslate">jdbc:h2:<url>;DB_CLOSE_ON_EXIT=FALSE </td> </tr> <tr> <td><a href="/misc/goto?guid=4958196678675012558">Execute SQL on connection</a></td> <td class="notranslate">jdbc:h2:<url>;INIT=RUNSCRIPT FROM '~/create.sql'<br /> jdbc:h2:file:~/sample;INIT=RUNSCRIPT FROM '~/create.sql'\\;RUNSCRIPT FROM '~/populate.sql'<br /> </td> </tr> <tr> <td><a href="/misc/goto?guid=4958196679420497535">User name and/or password</a></td> <td class="notranslate">jdbc:h2:<url>[;USER=<username>][;PASSWORD=<value>]<br /> jdbc:h2:file:~/sample;USER=sa;PASSWORD=123<br /> </td> </tr> <tr> <td><a href="/misc/goto?guid=4958196680160134489">Debug trace settings</a></td> <td class="notranslate">jdbc:h2:<url>;TRACE_LEVEL_FILE=<level 0..3><br /> jdbc:h2:file:~/sample;TRACE_LEVEL_FILE=3<br /> </td> </tr> <tr> <td><a href="/misc/goto?guid=4958196680895864570">Ignore unknown settings</a></td> <td class="notranslate">jdbc:h2:<url>;IGNORE_UNKNOWN_SETTINGS=TRUE<br /> </td> </tr> <tr> <td><a href="/misc/goto?guid=4958196681627008122">Custom file access mode</a></td> <td class="notranslate">jdbc:h2:<url>;ACCESS_MODE_DATA=rws<br /> </td> </tr> <tr> <td><a href="/misc/goto?guid=4958196682361083420">Database in a zip file</a></td> <td class="notranslate">jdbc:h2:zip:<zipFileName>!/<databaseName><br /> jdbc:h2:zip:~/db.zip!/test </td> </tr> <tr> <td><a href="/misc/goto?guid=4958196683102462280">Compatibility mode</a></td> <td class="notranslate">jdbc:h2:<url>;MODE=<databaseType><br /> jdbc:h2:~/test;MODE=MYSQL </td> </tr> <tr> <td><a href="/misc/goto?guid=4958196683839766672">Auto-reconnect</a></td> <td class="notranslate">jdbc:h2:<url>;AUTO_RECONNECT=TRUE<br /> jdbc:h2:tcp://localhost/~/test;AUTO_RECONNECT=TRUE </td> </tr> <tr> <td><a href="/misc/goto?guid=4958196671386438536">Automatic mixed mode</a></td> <td class="notranslate">jdbc:h2:<url>;AUTO_SERVER=TRUE<br /> jdbc:h2:~/test;AUTO_SERVER=TRUE </td> </tr> <tr> <td><a href="/misc/goto?guid=4958196685247216546">Page size</a></td> <td class="notranslate">jdbc:h2:<url>;PAGE_SIZE=512<br /> </td> </tr> <tr> <td><a href="/misc/goto?guid=4958196685991702903">Changing other settings</a></td> <td class="notranslate">jdbc:h2:<url>;<setting>=<value>[;<setting>=<value>...]<br /> jdbc:h2:file:~/sample;TRACE_LEVEL_SYSTEM_OUT=3<br /> </td> </tr> </tbody> </table> <h2 id="embedded_databases">Connecting to an Embedded (Local) Database</h2> <p>The database URL for connecting to a local database is <code class="notranslate">jdbc:h2:[file:][<path>]<databaseName></code>. The prefix <code class="notranslate">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 recommended to use lowercase letters only. The database name must be at least three characters long (a limitation of <code class="notranslate">File.createTempFile</code>). To point to the user home directory, use <code class="notranslate">~/</code>, as in: <code class="notranslate">jdbc:h2:~/test</code>. </p> <h2 id="in_memory_databases">In-Memory Databases</h2> <p>For certain use cases (for example: rapid prototyping, testing, high performance operations, read-only databases), it may not be required to persist data, or persist changes to the data. This database supports the in-memory mode, where the data is not persisted. </p> <p>In some cases, only one connection to a in-memory database is required. This means the database to be opened is private. In this case, the database URL is <code class="notranslate">jdbc:h2:mem:</code> Opening two connections within the same virtual machine means opening two different (private) databases. </p> <p>Sometimes multiple connections to the same in-memory database are required. In this case, the database URL must include a name. Example: <code class="notranslate">jdbc:h2:mem:db1</code>. Accessing the same database using this URL only works within the same virtual machine and class loader environment. </p> <p>To access an in-memory database from another process or from another computer, you need to start a TCP server in the same process as the in-memory database was created. The other processes then need to access the database over TCP/IP or SSL/TLS, using a database URL such as: <code class="notranslate">jdbc:h2:tcp://localhost/mem:db1</code>. </p> <p>By default, closing the last connection to a database closes the database. For an in-memory database, this means the content is lost. To keep the database open, add <code class="notranslate">;DB_CLOSE_DELAY=-1</code> to the database URL. To keep the content of an in-memory database as long as the virtual machine is alive, use <code class="notranslate">jdbc:h2:mem:test;DB_CLOSE_DELAY=-1</code>. </p> <h2 id="file_encryption">Database Files Encryption</h2> <p>The database files can be encrypted. Two encryption algorithms are supported: AES and XTEA. To use file encryption, you need to specify the encryption algorithm (the 'cipher') and the file password (in addition to the user password) when connecting to the database. </p> <h3>Creating a New Database with File Encryption</h3> <p>By default, a new database is automatically created if it does not exist yet. To create an encrypted database, connect to it as it would already exist. </p> <h3>Connecting to an Encrypted Database</h3> <p>The encryption algorithm is set in the database URL, and the file password is specified in the password field, before the user password. A single space separates the file password and the user password; the file password itself may not contain spaces. File passwords and user passwords are case sensitive. Here is an example to connect to a password-encrypted database: </p> <pre class="notranslate">Class.forName("org.h2.Driver"); String url = "jdbc:h2:~/test;CIPHER=AES"; String user = "sa"; String pwds = "filepwd userpwd"; conn = DriverManager. getConnection(url, user, pwds);</pre> <h3>Encrypting or Decrypting a Database</h3> <p>To encrypt an existing database, use the <code class="notranslate">ChangeFileEncryption</code> tool. This tool can also decrypt an encrypted database, or change the file encryption key. The tool is available from within the H2 Console in the tools section, or you can run it from the command line. The following command line will encrypt the database <code class="notranslate">test</code> in the user home directory with the file password <code class="notranslate">filepwd</code> and the encryption algorithm AES: </p> <pre class="notranslate">java -cp h2*.jar org.h2.tools.ChangeFileEncryption -dir ~ -db test -cipher AES -encrypt filepwd</pre> <h2 id="database_file_locking">Database File Locking</h2> <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. </p> <p>The following file locking methods are implemented: </p> <ul> <li>The default method is <code class="notranslate">FILE</code> and uses a watchdog thread to protect the database file. The watchdog reads the lock file each second. </li> <li>The second method is <code class="notranslate">SOCKET</code> 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 one (and always the same) computer. </li> <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. Failing to do so will result in a corrupted database. Using the method <code class="notranslate">NO</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.</li> </ul> <p>To open the database with a different file locking method, use the parameter <code class="notranslate">FILE_LOCK</code>. The following code opens the database with the 'socket' locking method: </p> <pre class="notranslate">String url = "jdbc:h2:~/test;FILE_LOCK=SOCKET";</pre> <p>For more information about the algorithms, see <a href="/misc/goto?guid=4958196686722786544">Advanced / File Locking Protocols</a>. </p> <h2 id="database_only_if_exists">Opening a Database Only if it Already Exists</h2> <p>By default, when an application calls <code class="notranslate">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 databases, and only allow to open existing databases. To do this, add <code class="notranslate">;IFEXISTS=TRUE</code> to the database 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: </p> <pre class="notranslate">String url = "jdbc:h2:/data/sample;IFEXISTS=TRUE";</pre> <h2 id="closing_a_database">Closing a Database</h2> <h3>Delayed Database Closing</h3> <p>Usually, a 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 to keep at least one connection open. The automatic closing of a database can be delayed or disabled with the SQL statement <code class="notranslate">SET DB_CLOSE_DELAY <seconds></code>. The parameter <seconds> specifies the number of seconds to keep a database open after the last connection to it was closed. The following statement will keep a database open for 10 seconds after the last connection was closed: </p> <pre class="notranslate">SET DB_CLOSE_DELAY 10</pre> <p>The value -1 means the database is not 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 class="notranslate">jdbc:h2:~/test;DB_CLOSE_DELAY=10</code>. </p> <h3 id="do_not_close_on_exit">Don't Close a Database when the VM Exits</h3> <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, 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: </p> <pre class="notranslate">String url = "jdbc:h2:~/test;DB_CLOSE_ON_EXIT=FALSE";</pre> <h2 id="execute_sql_on_connection">Execute SQL on Connection</h2> <p>Sometimes, particularly for in-memory databases, it is useful to be able to execute DDL or DML commands automatically when a client connects to a database. This functionality is enabled via the INIT property. Note that multiple commands may be passed to INIT, but the semicolon delimiter must be escaped, as in the example below. </p> <pre class="notranslate">String url = "jdbc:h2:mem;INIT=RUNSCRIPT FROM '~/create.sql'\\;RUNSCRIPT FROM '~/populate.sql'";</pre> <h2 id="ignore_unknown_settings">Ignore Unknown Settings</h2> <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 <code class="notranslate">PREFERDOSLIKELINEENDS</code> and <code class="notranslate">IGNOREDRIVERPRIVILEGES</code> 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 <code class="notranslate">;IGNORE_UNKNOWN_SETTINGS=TRUE</code> to the database URL. </p> <h2 id="other_settings">Changing Other Settings when Opening a Connection</h2> <p>In addition to the settings already described, other database settings can be passed in the database URL. Adding <code class="notranslate">;setting=value</code> at the end of a database URL is the same as executing the statement <code class="notranslate">SET setting value</code> just after connecting. For a list of supported settings, see <a href="/misc/goto?guid=4958196687462829976">SQL Grammar</a>. </p> <h2 id="custom_access_mode">Custom File Access Mode</h2> <p>Usually, the database opens the database file with the access mode <code class="notranslate">rw</code>, meaning read-write (except for read only databases, where the mode <code class="notranslate">r</code> is used). To open a database in read-only mode if the database file is not read-only, use <code class="notranslate">ACCESS_MODE_DATA=r</code>. Also supported are <code class="notranslate">rws</code> and <code class="notranslate">rwd</code>. This setting must be specified in the database URL: </p> <pre class="notranslate">String url = "jdbc:h2:~/test;ACCESS_MODE_DATA=rws";</pre> <p>For more information see <a href="/misc/goto?guid=4958196688204913851">Durability Problems</a>. On many operating systems the access mode <code class="notranslate">rws</code> does not guarantee that the data is written to the disk. </p> <h2 id="multiple_connections">Multiple Connections</h2> <h3>Opening Multiple Databases at the Same Time</h3> <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. </p> <h3>Multiple Connections to the Same Database: Client/Server</h3> <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). </p> <h3>Multithreading Support</h3> <p>This database is multithreading-safe. That means, if an application is multi-threaded, it does not need to worry about synchronizing access to the database. Internally, most requests to the same database are synchronized. That means an application can use multiple threads that access the same database at the same time, however if one thread executes a long running query, the other threads need to wait. </p> <p>An application should normally use one connection per thread. This database synchronizes access to the same connection, but other databases may not do this. </p> <h3>Locking, Lock-Timeout, Deadlocks</h3> <p>Unless <a href="/misc/goto?guid=4958196688940765500">multi-version concurrency</a> is used, 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). All locks are released when the transaction commits or rolls back. When using the default transaction isolation level 'read committed', read locks are already released after each statement. </p> <p>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 a connection cannot get a lock for a specified time, then a lock timeout exception is thrown. </p> <p>Usually, <code class="notranslate">SELECT</code> statements 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 <code class="notranslate">SELECT ... FOR UPDATE</code>. The statements <code class="notranslate">COMMIT</code> and <code class="notranslate">ROLLBACK</code> releases all open locks. The commands <code class="notranslate">SAVEPOINT</code> and <code class="notranslate">ROLLBACK TO SAVEPOINT</code> 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. The following statements generate locks: </p> <table class="main ke-zeroborder"> <tbody> <tr> <th>Type of Lock</th> <th>SQL Statement</th> </tr> <tr> <td>Read</td> <td class="notranslate">SELECT * FROM TEST;<br /> CALL SELECT MAX(ID) FROM TEST;<br /> SCRIPT;</td> </tr> <tr> <td>Write</td> <td class="notranslate">SELECT * FROM TEST WHERE 1=0 FOR UPDATE;</td> </tr> <tr> <td>Write</td> <td class="notranslate">INSERT INTO TEST VALUES(1, 'Hello');<br /> INSERT INTO TEST SELECT * FROM TEST;<br /> UPDATE TEST SET NAME='Hi';<br /> DELETE FROM TEST;</td> </tr> <tr> <td>Write</td> <td class="notranslate">ALTER TABLE TEST ...;<br /> CREATE INDEX ... ON TEST ...;<br /> DROP INDEX ...;</td> </tr> </tbody> </table> <p>The number of seconds until a lock timeout exception is thrown can be set separately for each connection using the SQL command <code class="notranslate">SET LOCK_TIMEOUT <milliseconds></code>. The initial lock timeout (that is the timeout used for new connections) can be set using the SQL command <code class="notranslate">SET DEFAULT_LOCK_TIMEOUT <milliseconds></code>. The default lock timeout is persistent. </p> <h3>Avoiding Deadlocks</h3> <p>To avoid deadlocks, ensure that all transactions lock the tables in the same order (for example in alphabetical order), and avoid upgrading read locks to write locks. Both can be achieved using explicitly locking tables using <code class="notranslate">SELECT ... FOR UPDATE</code>. </p> <h2 id="database_file_layout">Database File Layout</h2> <p>The following files are created for persistent databases: </p> <table class="main ke-zeroborder"> <tbody> <tr> <th>File Name</th> <th>Description</th> <th>Number of Files</th> </tr> <tr> <td class="notranslate">test.h2.db </td> <td>Database file.<br /> Contains the transaction log, indexes, and data for all tables.<br /> Format: <code class="notranslate"><database>.h2.db</code> </td> <td>1 per database </td> </tr> <tr> <td class="notranslate">test.lock.db </td> <td>Database lock file.<br /> Automatically (re-)created while the database is in use.<br /> Format: <code class="notranslate"><database>.lock.db</code> </td> <td>1 per database (only if in use) </td> </tr> <tr> <td class="notranslate">test.trace.db </td> <td>Trace file (if the trace option is enabled).<br /> Contains trace information.<br /> Format: <code class="notranslate"><database>.trace.db</code><br /> Renamed to <code class="notranslate"><database>.trace.db.old</code> is too big. </td> <td>0 or 1 per database </td> </tr> <tr> <td class="notranslate">test.lobs.db/* </td> <td>Directory containing one file for each<br /> BLOB or CLOB value larger than a certain size.<br /> Format: <code class="notranslate"><id>.t<tableId>.lob.db</code> </td> <td>1 per large object </td> </tr> <tr> <td class="notranslate">test.123.temp.db </td> <td>Temporary file.<br /> Contains a temporary blob or a large result set.<br /> Format: <code class="notranslate"><database>.<id>.temp.db</code> </td> <td>1 per object </td> </tr> </tbody> </table> <h3>Moving and Renaming Database Files</h3> <p>Database name and location are not stored inside the database files. </p> <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 of the same database start with the same name and the respective extensions are unchanged). </p> <p>As there is no platform specific data in the files, they can be moved to other operating systems without problems. </p> <h3>Backup</h3> <p>When the database is closed, it is possible to backup the database files. </p> <p>To backup data while the database is running, the SQL commands <code class="notranslate">SCRIPT</code> and <code class="notranslate">BACKUP</code> can be used. </p> <h2 id="logging_recovery">Logging and Recovery</h2> <p>Whenever data is modified in the database and those changes are committed, the changes are written to the transaction log (except for in-memory objects). The changes to the main data area itself are usually written later on, to optimize disk access. If there is a power failure, the main data area is not up-to-date, but because the changes are in the transaction log, the next time the database is opened, the changes are re-applied automatically. </p> <h2 id="compatibility">Compatibility</h2> <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: </p> <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 <code class="notranslate">IGNORECASE=TRUE</code> to the database URL (example: <code class="notranslate">jdbc:h2:~/test;IGNORECASE=TRUE</code>). </p> <h3>Compatibility Modes</h3> <p>For certain features, this database can emulate the behavior of specific databases. Not all features or differences of those databases are implemented. Here is the list of currently supported modes and the differences to the regular mode: </p> <h3>DB2 Compatibility Mode</h3> <p>To use the IBM DB2 mode, use the database URL <code class="notranslate">jdbc:h2:~/test;MODE=DB2</code> or the SQL statement <code class="notranslate">SET MODE DB2</code>. </p> <ul> <li>For aliased columns, <code class="notranslate">ResultSetMetaData.getColumnName()</code> returns the alias name and <code class="notranslate">getTableName()</code> returns <code class="notranslate">null</code>. </li> <li>Support for the syntax <code class="notranslate">[OFFSET .. ROW] [FETCH ... ONLY]</code> as an alternative for <code class="notranslate">LIMIT .. OFFSET</code>. </li> <li>Concatenating <code class="notranslate">NULL</code> with another value results in the other value. </li> <li>Support the pseudo-table SYSIBM.SYSDUMMY1. </li> </ul> <h3>Derby Compatibility Mode</h3> <p>To use the Apache Derby mode, use the database URL <code class="notranslate">jdbc:h2:~/test;MODE=Derby</code> or the SQL statement <code class="notranslate">SET MODE Derby</code>. </p> <ul> <li>For aliased columns, <code class="notranslate">ResultSetMetaData.getColumnName()</code> returns the alias name and <code class="notranslate">getTableName()</code> returns <code class="notranslate">null</code>. </li> <li>For unique indexes, <code class="notranslate">NULL</code> is distinct. That means only one row with <code class="notranslate">NULL</code> in one of the columns is allowed. </li> <li>Concatenating <code class="notranslate">NULL</code> with another value results in the other value. </li> <li>Support the pseudo-table SYSIBM.SYSDUMMY1. </li> </ul> <h3>HSQLDB Compatibility Mode</h3> <p>To use the HSQLDB mode, use the database URL <code class="notranslate">jdbc:h2:~/test;MODE=HSQLDB</code> or the SQL statement <code class="notranslate">SET MODE HSQLDB</code>. </p> <ul> <li>For aliased columns, <code class="notranslate">ResultSetMetaData.getColumnName()</code> returns the alias name and <code class="notranslate">getTableName()</code> returns <code class="notranslate">null</code>. </li> <li>When converting the scale of decimal data, the number is only converted if the new scale is smaller than the current scale. Usually, the scale is converted and 0s are added if required. </li> <li>For unique indexes, <code class="notranslate">NULL</code> is distinct. That means only one row with <code class="notranslate">NULL</code> in one of the columns is allowed. </li> <li>Text can be concatenated using '+'. </li> </ul> <h3>MS SQL Server Compatibility Mode</h3> <p>To use the MS SQL Server mode, use the database URL <code class="notranslate">jdbc:h2:~/test;MODE=MSSQLServer</code> or the SQL statement <code class="notranslate">SET MODE MSSQLServer</code>. </p> <ul> <li>For aliased columns, <code class="notranslate">ResultSetMetaData.getColumnName()</code> returns the alias name and <code class="notranslate">getTableName()</code> returns <code class="notranslate">null</code>. </li> <li>Identifiers may be quoted using square brackets as in <code class="notranslate">[Test]</code>. </li> <li>For unique indexes, <code class="notranslate">NULL</code> is distinct. That means only one row with <code class="notranslate">NULL</code> in one of the columns is allowed. </li> <li>Concatenating <code class="notranslate">NULL</code> with another value results in the other value. </li> <li>Text can be concatenated using '+'. </li> </ul> <h3>MySQL Compatibility Mode</h3> <p>To use the MySQL mode, use the database URL <code class="notranslate">jdbc:h2:~/test;MODE=MySQL</code> or the SQL statement <code class="notranslate">SET MODE MySQL</code>. </p> <ul> <li>When inserting data, if a column is defined to be <code class="notranslate">NOT NULL</code> and <code class="notranslate">NULL</code> 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. </li> <li>Creating indexes in the <code class="notranslate">CREATE TABLE</code> statement is allowed using <code class="notranslate">INDEX(..)</code> or <code class="notranslate">KEY(..)</code>. Example: <code class="notranslate">create table test(id int primary key, name varchar(255), key idx_name(name));</code> </li> <li>Meta data calls return identifiers in lower case. </li> <li>When converting a floating point number to an integer, the fractional digits are not truncated, but the value is rounded. </li> <li>Concatenating <code class="notranslate">NULL</code> with another value results in the other value. </li> </ul> <p>Text comparison in MySQL is case insensitive by default, while in H2 it is case sensitive (as in most other databases). H2 does support case insensitive text comparison, but it needs to be set separately, using <code class="notranslate">SET IGNORECASE TRUE</code>. This affects comparison using <code class="notranslate">=, LIKE, REGEXP</code>. </p> <h3>Oracle Compatibility Mode</h3> <p>To use the Oracle mode, use the database URL <code class="notranslate">jdbc:h2:~/test;MODE=Oracle</code> or the SQL statement <code class="notranslate">SET MODE Oracle</code>. </p> <ul> <li>For aliased columns, <code class="notranslate">ResultSetMetaData.getColumnName()</code> returns the alias name and <code class="notranslate">getTableName()</code> returns <code class="notranslate">null</code>. </li> <li>When using unique indexes, multiple rows with <code class="notranslate">NULL</code> in all columns are allowed, however it is not allowed to have multiple rows with the same values otherwise. </li> <li>Concatenating <code class="notranslate">NULL</code> with another value results in the other value. </li> </ul> <h3>PostgreSQL Compatibility Mode</h3> <p>To use the PostgreSQL mode, use the database URL <code class="notranslate">jdbc:h2:~/test;MODE=PostgreSQL</code> or the SQL statement <code class="notranslate">SET MODE PostgreSQL</code>. </p> <ul> <li>For aliased columns, <code class="notranslate">ResultSetMetaData.getColumnName()</code> returns the alias name and <code class="notranslate">getTableName()</code> returns <code class="notranslate">null</code>. </li> <li>When converting a floating point number to an integer, the fractional digits are not be truncated, but the value is rounded. </li> <li>The system columns <code class="notranslate">CTID</code> and <code class="notranslate">OID</code> are supported. </li> </ul> <h2 id="auto_reconnect">Auto-Reconnect</h2> <p>The auto-reconnect feature causes the JDBC driver to reconnect to the database if the connection is lost. The automatic re-connect only occurs when auto-commit is enabled; if auto-commit is disabled, an exception is thrown. To enable this mode, append <code class="notranslate">;AUTO_RECONNECT=TRUE</code> to the database URL. </p> <p>Re-connecting will open a new session. After an automatic re-connect, variables and local temporary tables definitions (excluding data) are re-created. The contents of the system table <code class="notranslate">INFORMATION_SCHEMA.SESSION_STATE</code> contains all client side state that is re-created. </p> <p>If another connection uses the database in exclusive mode (enabled using <code class="notranslate">SET EXCLUSIVE 1</code> or <code class="notranslate">SET EXCLUSIVE 2</code>), then this connection will try to re-connect until the exclusive mode ends. </p> <h2 id="auto_mixed_mode">Automatic Mixed Mode</h2> <p>Multiple processes can access the same database without having to start the server manually. To do that, append <code class="notranslate">;AUTO_SERVER=TRUE</code> to the database URL. You can use the same database URL independent of whether the database is already open or not. This feature doesn't work with in-memory databases. Example database URL: </p> <pre class="notranslate">jdbc:h2:/data/test;AUTO_SERVER=TRUE</pre> <p>Use the same URL for all connections to this database. Internally, when using this mode, the first connection to the database is made in embedded mode, and additionally a server is started internally (as a daemon thread). If the database is already open in another process, the server mode is used automatically. The IP address and port of the server are stored in the file <code class="notranslate">.lock.db</code>, that's why in-memory databases can't be supported. </p> <p>The application that opens the first connection to the database uses the embedded mode, which is faster than the server mode. Therefore the main application should open the database first if possible. The first connection automatically starts a server on a random port. This server allows remote connections, however only to this database (to ensure that, the client reads <code class="notranslate">.lock.db</code> file and sends the the random key that is stored there to the server). When the first connection is closed, the server stops. If other (remote) connections are still open, one of them will then start a server (auto-reconnect is enabled automatically). </p> <p>All processes need to have access to the database files. If the first connection is closed (the connection that started the server), open transactions of other connections will be rolled back (this may not be a problem if you don't disable autocommit). Explicit client/server connections (using <code class="notranslate">jdbc:h2:tcp://</code> or <code class="notranslate">ssl://</code>) are not supported. This mode is not supported for in-memory databases. </p> <p>Here is an example how to use this mode. Application 1 and 2 are not necessarily started on the same computer, but they need to have access to the database files. Application 1 and 2 are typically two different processes (however they could run within the same process). </p> <pre class="notranslate">// Application 1: DriverManager.getConnection("jdbc:h2:/data/test;AUTO_SERVER=TRUE"); // Application 2: DriverManager.getConnection("jdbc:h2:/data/test;AUTO_SERVER=TRUE");</pre> <h2 id="page_size">Page Size</h2> <p>The page size for new databases is 2 KB (2048), unless the page size is set explicitly in the database URL using <code class="notranslate">PAGE_SIZE=</code> when the database is created. The page size of existing databases can not be changed, so this property needs to be set when the database is created. </p> <h2 id="trace_options">Using the Trace Options</h2> <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: </p> <ul> <li>Trace to <code class="notranslate">System.out</code> and/or to a file </li> <li>Support for trace levels <code class="notranslate">OFF, ERROR, INFO, DEBUG</code> </li> <li>The maximum size of the trace file can be set </li> <li>It is possible to generate Java source code from the trace file </li> <li>Trace can be enabled at runtime by manually creating a file </li> </ul> <h3>Trace Options</h3> <p>The simplest way to enable the trace option is setting it in the database URL. There are two settings, one for <code class="notranslate">System.out</code> (<code class="notranslate">TRACE_LEVEL_SYSTEM_OUT</code>) tracing, and one for file tracing (<code class="notranslate">TRACE_LEVEL_FILE</code>). The trace levels are 0 for <code class="notranslate">OFF</code>, 1 for <code class="notranslate">ERROR</code> (the default), 2 for <code class="notranslate">INFO</code>, and 3 for <code class="notranslate">DEBUG</code>. A database URL with both levels set to <code class="notranslate">DEBUG</code> is: </p> <pre class="notranslate">jdbc:h2:~/test;TRACE_LEVEL_FILE=3;TRACE_LEVEL_SYSTEM_OUT=3</pre> <p>The trace level can be changed at runtime by executing the SQL command <code class="notranslate">SET TRACE_LEVEL_SYSTEM_OUT level</code> (for <code class="notranslate">System.out</code> tracing) or <code class="notranslate">SET TRACE_LEVEL_FILE level</code> (for file tracing). Example: </p> <pre class="notranslate">SET TRACE_LEVEL_SYSTEM_OUT 3</pre> <h3>Setting the Maximum Size of the Trace File</h3> <p>When using a high trace level, the trace file can get very big quickly. The default size limit is 16 MB, if the trace file exceeds this limit, it is renamed to <code class="notranslate">.old</code> and a new file is created. If another such file exists, it is deleted. To limit the size to a certain number of megabytes, use <code class="notranslate">SET TRACE_MAX_FILE_SIZE mb</code>. Example: </p> <pre class="notranslate">SET TRACE_MAX_FILE_SIZE 1</pre> <h3>Java Code Generation</h3> <p>When setting the trace level to <code class="notranslate">INFO</code> or <code class="notranslate">DEBUG</code>, Java source code is generated as well. This simplifies reproducing problems. The trace file looks like this: </p> <pre class="notranslate">... 12-20 20:58:09 jdbc[0]: /**/dbMeta3.getURL(); 12-20 20:58:09 jdbc[0]: /**/dbMeta3.getTables(null, "", null, new String[]{"TABLE", "VIEW"}); ...</pre> <p>To filter the Java source code, use the <code class="notranslate">ConvertTraceFile</code> tool as follows: </p> <pre class="notranslate">java -cp h2*.jar org.h2.tools.ConvertTraceFile -traceFile "~/test.trace.db" -javaClass "Test"</pre> <p>The generated file <code class="notranslate">Test.java</code> will contain the Java source code. The generated source code may be too large to compile (the size of a Java method is limited). If this is the case, the source code needs to be split in multiple methods. The password is not listed in the trace file and therefore not included in the source code. </p> <h2 id="other_logging">Using Other Logging APIs</h2> <p>By default, this database uses its own native 'trace' facility. This facility is called 'trace' and not 'log' within this database to avoid confusion with the transaction log. Trace messages can be written to both file and <code class="notranslate">System.out</code>. In most cases, this is sufficient, however sometimes it is better to use the same facility as the application, for example Log4j. To do that, this database support SLF4J. </p> <p><a href="/misc/goto?guid=4958196689669996176">SLF4J</a> is a simple facade for various logging APIs and allows to plug in the desired implementation at deployment time. SLF4J supports implementations such as Logback, Log4j, Jakarta Commons Logging (JCL), Java logging, x4juli, and Simple Log. </p> <p>To enable SLF4J, set the file trace level to 4 in the database URL: </p> <pre class="notranslate">jdbc:h2:~/test;TRACE_LEVEL_FILE=4</pre> <p>Changing the log mechanism is not possible after the database is open, that means executing the SQL statement <code class="notranslate">SET TRACE_LEVEL_FILE 4</code> when the database is already open will not have the desired effect. To use SLF4J, all required jar files need to be in the classpath. If it does not work, check the file <code class="notranslate"><database>.trace.db</code> for error messages. </p> <h2 id="read_only">Read Only Databases</h2> <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 <code class="notranslate">SELECT</code> and <code class="notranslate">CALL</code> statements are allowed. To create a read-only database, close the database. Then, make the database file read-only. When you open the database now, it is read-only. There are two ways an application can find out whether database is read-only: by calling <code class="notranslate">Connection.isReadOnly()</code> or by executing the SQL statement <code class="notranslate">CALL READONLY()</code>. </p> <p>Using the <a href="/misc/goto?guid=4958196681627008122">Custom Access Mode</a> <code class="notranslate">r</code> the database can also be opened in read-only mode, even if the database file is not read only. </p> <h2 id="database_in_zip">Read Only Databases in Zip or Jar File</h2> <p>To create a read-only database in a zip file, first create a regular persistent database, and then create a backup. The database must not have pending changes, that means you need to close all connections to the database first. To speed up opening the read-only database and running queries, the database should be closed using <code class="notranslate">SHUTDOWN DEFRAG</code>. If you are using a database named <code class="notranslate">test</code>, an easy way to create a zip file is using the <code class="notranslate">Backup</code> tool. You can start the tool from the command line, or from within the H2 Console (Tools - Backup). Please note that the database must be closed when the backup is created. Therefore, the SQL statement <code class="notranslate">BACKUP TO</code> can not be used. </p> <p>When the zip file is created, you can open the database in the zip file using the following database URL: </p> <pre class="notranslate">jdbc:h2:zip:~/data.zip!/test</pre> <p>Databases in zip files 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; therefore large databases are supported as well. The same indexes are used as when using a regular database. </p> <p>If the database is larger than a few megabytes, performance is much better if the database file is split into multiple smaller files, because random access in compressed files is not possible. See also the sample application <a href="/misc/goto?guid=4958196691074582608">ReadOnlyDatabaseInZip</a>. </p> <h2 id="low_disk_space">Graceful Handling of Low Disk Space Situations</h2> <p>If the database needs more disk space, it calls the database event listener if one is installed. The application may then delete temporary files, or display a message and wait until the user has resolved the problem. To install a listener, run the SQL statement <code class="notranslate">SET DATABASE_EVENT_LISTENER</code> or use a database URL of the form <code class="notranslate">jdbc:h2:~/test;DATABASE_EVENT_LISTENER='com.acme.DbListener'</code> (the quotes around the class name are required). See also the <code class="notranslate">DatabaseEventListener</code> API. </p> <h3>Opening a Corrupted Database</h3> <p>If a database cannot 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. </p> <h2 id="computed_columns">Computed Columns / Function Based Index</h2> <p>Function indexes are not directly supported by this database, but they can be emulated by using computed columns. For example, if an index on the upper-case version of a column is required, create a computed column with the upper-case version of the original column, and create an index for this column: </p> <pre class="notranslate">CREATE TABLE ADDRESS( ID INT PRIMARY KEY, NAME VARCHAR, UPPER_NAME VARCHAR AS UPPER(NAME) ); CREATE INDEX IDX_U_NAME ON ADDRESS(UPPER_NAME);</pre> <p>When inserting data, it is not required (and 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: </p> <pre class="notranslate">INSERT INTO ADDRESS(ID, NAME) VALUES(1, 'Miller'); SELECT * FROM ADDRESS WHERE UPPER_NAME='MILLER';</pre> <h2 id="multi_dimensional">Multi-Dimensional Indexes</h2> <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. </p> <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). </p> <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 <code class="notranslate">TestMultiDimension.java</code>. </p> <h2 id="user_defined_functions">User-Defined Functions and Stored Procedures</h2> <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. A function can be defined using source code, or as a reference to a compiled class that is available in the classpath. By default, the function aliases are stored in the current schema. </p> <h3>Referencing a Compiled Method</h3> <p>When referencing a method, the class must already be compiled and included in the classpath where the database is running. Only static Java methods are supported; both the class and the method must be public. Example Java class: </p> <pre class="notranslate">package acme; import java.math.*; public class Function { public static boolean isPrime(int value) { return new BigInteger(String.valueOf(value)).isProbablePrime(100); } }</pre> <p>The Java function must be registered in the database by calling <code class="notranslate">CREATE ALIAS ... FOR</code>: </p> <pre class="notranslate">CREATE ALIAS IS_PRIME FOR "acme.Function.isPrime";</pre> <p>For a complete sample application, see <code class="notranslate">src/test/org/h2/samples/Function.java</code>. </p> <h3>Declaring Functions as Source Code</h3> <p>When defining a function alias with source code, the database tries to compile the source code using the Sun Java compiler (the class <code class="notranslate">com.sun.tools.javac.Main</code>) if the <code class="notranslate">tools.jar</code> is in the classpath. If not, <code class="notranslate">javac</code> is run as a separate process. Only the source code is stored in the database; the class is compiled each time the database is re-opened. Source code is usually passed as dollar quoted text to avoid escaping problems, however single quotes can be used as well. Example: </p> <pre class="notranslate">CREATE ALIAS NEXT_PRIME AS $$ String nextPrime(String value) { return new BigInteger(value).nextProbablePrime().toString(); } $$;</pre> <p>By default, the three packages <code class="notranslate">java.util, java.math, java.sql</code> are imported. The method name (<code class="notranslate">nextPrime</code> in the example above) is ignored. Method overloading is not supported when declaring functions as source code, that means only one method may be declared for an alias. If different import statements are required, they must be declared at the beginning and separated with the tag <code class="notranslate">@CODE</code>: </p> <pre class="notranslate">CREATE ALIAS IP_ADDRESS AS $$ import java.net.*; @CODE String ipAddress(String host) throws Exception { return InetAddress.getByName(host).getHostAddress(); } $$;</pre> <p>The following template is used to create a complete Java class: </p> <pre class="notranslate">package org.h2.dynamic; < import statements before the tag @CODE; if not set: import java.util.*; import java.math.*; import java.sql.*; > public class <aliasName> { public static <sourceCode> }</pre> <h3>Method Overloading</h3> <p>Multiple methods may be bound to a SQL function if the class is already compiled and included in the classpath. Each Java method must have a different number of arguments. Method overloading is not supported when declaring functions as source code. </p> <h3>Function Data Type Mapping</h3> <p>Functions that accept non-nullable parameters such as <code class="notranslate">int</code> will not be called if one of those parameters is <code class="notranslate">NULL</code>. Instead, the result of the function is <code class="notranslate">NULL</code>. If the function should be called if a parameter is <code class="notranslate">NULL</code>, you need to use <code class="notranslate">java.lang.Integer</code> instead. </p> <p>SQL types are mapped to Java classes and vice-versa as in the JDBC API. For details, see <a href="/misc/goto?guid=4958196691806153134">Data Types</a>. There are a few special cases: <code class="notranslate">java.lang.Object</code> is mapped to <code class="notranslate">OTHER</code> (a serialized object). Therefore, <code class="notranslate">java.lang.Object</code> can not be used to match all SQL types (matching all SQL types is not supported). The second special case is <code class="notranslate">Object[]</code>: arrays of any class are mapped to <code class="notranslate">ARRAY</code>. Objects of type <code class="notranslate">org.h2.value.Value</code> (the internal value class) are passed through without conversion. </p> <h3>Functions That Require a Connection</h3> <p>If the first parameter of a Java function is a <code class="notranslate">java.sql.Connection</code>, then the connection to database is provided. This connection does not need to be closed before returning. When calling the method from within the SQL statement, this connection parameter does not need to be (can not be) specified. </p> <h3>Functions Throwing an Exception</h3> <p>If a function throws an exception, then the current statement is rolled back and the exception is thrown to the application. SQLException are directly re-thrown to the calling application; all other exceptions are first converted to a SQLException. </p> <h3>Functions Returning a Result Set</h3> <p>Functions may returns a result set. Such a function can be called with the <code class="notranslate">CALL</code> statement: </p> <pre class="notranslate">public static ResultSet query(Connection conn, String sql) throws SQLException { return conn.createStatement().executeQuery(sql); } CREATE ALIAS QUERY FOR "org.h2.samples.Function.query"; CALL QUERY('SELECT * FROM TEST');</pre> <h3>Using SimpleResultSet</h3> <p>A function can create a result set using the <code class="notranslate">SimpleResultSet</code> tool: </p> <pre class="notranslate">import org.h2.tools.SimpleResultSet; ... public static ResultSet simpleResultSet() throws SQLException { SimpleResultSet rs = new SimpleResultSet(); rs.addColumn("ID", Types.INTEGER, 10, 0); rs.addColumn("NAME", Types.VARCHAR, 255, 0); rs.addRow(0, "Hello"); rs.addRow(1, "World"); return rs; } CREATE ALIAS SIMPLE FOR "org.h2.samples.Function.simpleResultSet"; CALL SIMPLE();</pre> <h3>Using a Function as a Table</h3> <p>A function that returns a result set can be used 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 <code class="notranslate">null</code> where not known at compile time). And then, while executing the statement to get the data (maybe multiple times 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 <code class="notranslate">jdbc:columnlist:connection</code>. Otherwise, the URL of the connection is <code class="notranslate">jdbc:default:connection</code>. </p> <pre class="notranslate">public static ResultSet getMatrix(Connection conn, Integer size) throws SQLException { SimpleResultSet rs = new SimpleResultSet(); rs.addColumn("X", Types.INTEGER, 10, 0); rs.addColumn("Y", Types.INTEGER, 10, 0); String url = conn.getMetaData().getURL(); if (url.equals("jdbc:columnlist:connection")) { return rs; } for (int s = size.intValue(), x = 0; x < s; x++) { for (int y = 0; y < s; y++) { rs.addRow(x, y); } } return rs; } CREATE ALIAS MATRIX FOR "org.h2.samples.Function.getMatrix"; SELECT * FROM MATRIX(4) ORDER BY X, Y;</pre> <h2 id="triggers">Triggers</h2> <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 <code class="notranslate">src/test/org/h2/samples/TriggerSample.java</code>. A Java trigger must implement the interface <code class="notranslate">org.h2.api.Trigger</code>. The trigger class must be available in the classpath of the database engine (when using the server mode, it must be in the classpath of the server). </p> <pre class="notranslate">import org.h2.api.Trigger; ... public class TriggerSample implements Trigger { public void init(Connection conn, String schemaName, String triggerName, String tableName, boolean before, int type) { // initialize the trigger object is necessary } public void fire(Connection conn, Object[] oldRow, Object[] newRow) throws SQLException { // the trigger is fired } public void close() { // the database is closed } public void remove() { // the trigger was dropped } }</pre> <p>The connection can be used to query or update data in other tables. The trigger then needs to be defined in the database: </p> <pre class="notranslate">CREATE TRIGGER INV_INS AFTER INSERT ON INVOICE FOR EACH ROW CALL "org.h2.samples.TriggerSample"</pre> <p>The trigger can be used to veto a change by throwing a <code class="notranslate">SQLException</code>. </p> <p>As an alternative to implementing the <code class="notranslate">Trigger</code> interface, an application can extend the abstract class <code class="notranslate">org.h2.tools.TriggerAdapter</code>. This will allows to use the <code class="notranslate">ResultSet</code> interface within trigger implementations. In this case, only the <code class="notranslate">fire</code> method needs to be implemented: </p> <pre class="notranslate">import org.h2.tools.TriggerAdapter; ... public class TriggerSample implements TriggerAdapter { public void fire(Connection conn, ResultSet oldRow, ResultSet newRow) throws SQLException { // the trigger is fired } }</pre> <h2 id="compacting">Compacting a Database</h2> <p>Empty space in the database file re-used automatically. When closing the database, the database is automatically compacted for up to 200 milliseconds by default. To compact more, use the SQL statement SHUTDOWN COMPACT. However re-creating the database may further reduce the database size because this will re-build the indexes. Here is a sample function to do this: </p> <pre class="notranslate">public static void compact(String dir, String dbName, String user, String password) throws Exception { String url = "jdbc:h2:" + dir + "/" + dbName; String file = "data/test.sql"; Script.execute(url, user, password, file); DeleteDbFiles.execute(dir, dbName, true); RunScript.execute(url, user, password, file, null, false); }</pre> <p>See also the sample application <code class="notranslate">org.h2.samples.Compact</code>. The commands <code class="notranslate">SCRIPT / RUNSCRIPT</code> can be used as well to create a backup of a database and re-build the database from the script. </p> <h2 id="cache_settings">Cache Settings</h2> <p>The database keeps most frequently used data in the main memory. The amount of memory used for caching can be changed using the setting <code class="notranslate">CACHE_SIZE</code>. This setting can be set in the database connection URL (<code class="notranslate">jdbc:h2:~/test;CACHE_SIZE=131072</code>), or it can be changed at runtime using <code class="notranslate">SET CACHE_SIZE size</code>. The size of the cache, as represented by <code class="notranslate">CACHE_SIZE</code> is measured in KB, with each KB being 1024 bytes. This setting has no effect for in-memory databases. For persistent databases, the setting is stored in the database and re-used when the database is opened the next time. However, when opening an existing database, the cache size is set to at most half the amount of memory available for the virtual machine (Runtime.getRuntime().maxMemory()), even if the cache size setting stored in the database is larger; however the setting stored in the database is kept. Setting the cache size in the database URL or explicitly using <code class="notranslate">SET CACHE_SIZE</code> overrides this value (even if larger than the physical memory). To get the current used maximum cache size, use the query <code class="notranslate">SELECT * FROM INFORMATION_SCHEMA.SETTINGS WHERE NAME = 'info.CACHE_MAX_SIZE'</code> </p> <p>An experimental scan-resistant cache algorithm "Two Queue" (2Q) is available. To enable it, append <code class="notranslate">;CACHE_TYPE=TQ</code> to the database URL. The cache might not actually improve performance. If you plan to use it, please run your own test cases first. </p> <p>Also included is an experimental second level soft reference cache. Rows in this cache are only garbage collected on low memory. By default the second level cache is disabled. To enable it, use the prefix <code class="notranslate">SOFT_</code>. Example: <code class="notranslate">jdbc:h2:~/test;CACHE_TYPE=SOFT_LRU</code>. The cache might not actually improve performance. If you plan to use it, please run your own test cases first. </p> <p>To get information about page reads and writes, and the current caching algorithm in use, call <code class="notranslate">SELECT * FROM INFORMATION_SCHEMA.SETTINGS</code>. The number of pages read / written is listed. </p> <p> </p>