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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"><html><head><title>SQLite Query Language: VACUUM</title><style type="text/css">body { margin: auto; font-family: "Verdana" "sans-serif"; padding: 8px 1%;}a { color: #45735f }a:visited { color: #734559 }.logo { position:absolute; margin:3px; }.tagline { float:right; text-align:right; font-style:italic; width:240px; margin:12px; margin-top:58px;}.toolbar { font-variant: small-caps; text-align: center; line-height: 1.6em; margin: 0; padding:1px 8px;}.toolbar a { color: white; text-decoration: none; padding: 6px 12px; }.toolbar a:visited { color: white; }.toolbar a:hover { color: #80a796; background: white; }.content { margin: 5%; }.content dt { font-weight:bold; }.content dd { margin-bottom: 25px; margin-left:20%; }.content ul { padding:0px; padding-left: 15px; margin:0px; }/* rounded corners */.se { background: url(images/se.png) 100% 100% no-repeat #80a796}.sw { background: url(images/sw.png) 0% 100% no-repeat }.ne { background: url(images/ne.png) 100% 0% no-repeat }.nw { background: url(images/nw.png) 0% 0% no-repeat }</style><meta http-equiv="content-type" content="text/html; charset=UTF-8"> </head><body><div><!-- container div to satisfy validator --><a href="index.html"><img class="logo" src="images/SQLite.gif" alt="SQLite Logo" border="0"></a><div><!-- IE hack to prevent disappearing logo--></div><div class="tagline">Small. Fast. Reliable.<br>Choose any three.</div><table width=100% style="clear:both"><tr><td> <div class="se"><div class="sw"><div class="ne"><div class="nw"> <div class="toolbar"> <a href="about.html">About</a> <a href="sitemap.html">Sitemap</a> <a href="docs.html">Documentation</a> <a href="download.html">Download</a> <a href="copyright.html">License</a> <a href="news.html">News</a> <a href="http://www.sqlite.org/cvstrac/index">Developers</a> <a href="support.html">Support</a> </div></div></div></div></div></td></tr></table> <a href="lang.html"><h2>SQL As Understood By SQLite</h2></a><h3>VACUUM</h3><table cellpadding="10"><tr><td align="right" valign="top"><i><font color="#ff3434">sql-statement</font></i> ::=</td><td><b><font color="#2c2cf0">VACUUM</font></b></td></tr></table><p>The VACUUM command is an SQLite extension modeled after a similarcommand found in PostgreSQL. If VACUUM is invoked with the name of atable or index then it is suppose to clean up the named table or index.In version 1.0 of SQLite, the VACUUM command would invoke <b>gdbm_reorganize()</b> to clean up the backend database file.</p><p>VACUUM became a no-op when the GDBM backend was removed fromSQLITE in version 2.0.0.VACUUM was reimplemented in version 2.8.1.</p><p>When an object (table, index, or trigger) is dropped from the database, it leaves behind empty space. This makes the database file larger than it needs to be, but can speed up inserts. In time inserts and deletes can leave the database file structure fragmented, which slows down disk access to the database contents.The VACUUM command cleansthe main database by copying its contents to a temporary database file and reloading the original database file from the copy. This eliminates free pages, aligns table data to be contiguous, and otherwise cleans up the database file structure.</p><p>The VACUUM command may change the <a href="lang_createtable.html#rowid">ROWID</a> of entires in tables that donot have an explicit INTEGER PRIMARY KEY.</p><p>VACUUM only works on the main database.It is not possible to VACUUM an attached database file.</p><p>The VACUUM command will fail if there is an active transaction.The VACUUM command is a no-op for in-memory databases.</p><p>As of SQLite version 3.1, an alternative to using the VACUUM commandis auto-vacuum mode, enabled using the <a href="pragma.html#pragma_auto_vacuum">auto_vacuum pragma</a>.When auto-vacuum is enabled for a database, large deletes causethe size of the database file to shrink. However, auto-vacuumalso causes excess fragmentation of the database file. And auto-vacuumdoes not compact partially filled pages of the database as VACUUMdoes.</p><DIV class="pdf_section"><hr><small><i>This page last modified 2008/04/15 19:31:05 UTC</i></small></div></body></html>⌨️ 快捷键说明
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