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diff --git a/www/lang_vacuum.html b/www/lang_vacuum.html new file mode 100644 index 0000000..539402f --- /dev/null +++ b/www/lang_vacuum.html @@ -0,0 +1,299 @@ +<!DOCTYPE html> +<html><head> +<meta name="viewport" content="width=device-width, initial-scale=1.0"> +<meta http-equiv="content-type" content="text/html; charset=UTF-8"> +<link href="sqlite.css" rel="stylesheet"> +<title>VACUUM</title> +<!-- path= --> +</head> +<body> +<div class=nosearch> +<a href="index.html"> +<img class="logo" src="images/sqlite370_banner.gif" alt="SQLite" border="0"> +</a> +<div><!-- IE hack to prevent disappearing logo --></div> +<div class="tagline desktoponly"> +Small. Fast. Reliable.<br>Choose any three. +</div> +<div class="menu mainmenu"> +<ul> +<li><a href="index.html">Home</a> +<li class='mobileonly'><a href="javascript:void(0)" onclick='toggle_div("submenu")'>Menu</a> +<li class='wideonly'><a href='about.html'>About</a> +<li class='desktoponly'><a href="docs.html">Documentation</a> +<li class='desktoponly'><a href="download.html">Download</a> +<li class='wideonly'><a href='copyright.html'>License</a> +<li class='desktoponly'><a href="support.html">Support</a> +<li class='desktoponly'><a href="prosupport.html">Purchase</a> +<li class='search' id='search_menubutton'> +<a href="javascript:void(0)" onclick='toggle_search()'>Search</a> +</ul> +</div> +<div class="menu submenu" id="submenu"> +<ul> +<li><a href='about.html'>About</a> +<li><a href='docs.html'>Documentation</a> +<li><a href='download.html'>Download</a> +<li><a href='support.html'>Support</a> +<li><a href='prosupport.html'>Purchase</a> +</ul> +</div> +<div class="searchmenu" id="searchmenu"> +<form method="GET" action="search"> +<select name="s" id="searchtype"> +<option value="d">Search Documentation</option> +<option value="c">Search Changelog</option> +</select> +<input type="text" name="q" id="searchbox" value=""> +<input type="submit" value="Go"> +</form> +</div> +</div> +<script> +function toggle_div(nm) { +var w = document.getElementById(nm); +if( w.style.display=="block" ){ +w.style.display = "none"; +}else{ +w.style.display = "block"; +} +} +function toggle_search() { +var w = document.getElementById("searchmenu"); +if( w.style.display=="block" ){ +w.style.display = "none"; +} else { +w.style.display = "block"; +setTimeout(function(){ +document.getElementById("searchbox").focus() +}, 30); +} +} +function div_off(nm){document.getElementById(nm).style.display="none";} +window.onbeforeunload = function(e){div_off("submenu");} +/* Disable the Search feature if we are not operating from CGI, since */ +/* Search is accomplished using CGI and will not work without it. */ +if( !location.origin || !location.origin.match || !location.origin.match(/http/) ){ +document.getElementById("search_menubutton").style.display = "none"; +} +/* Used by the Hide/Show button beside syntax diagrams, to toggle the */ +function hideorshow(btn,obj){ +var x = document.getElementById(obj); +var b = document.getElementById(btn); +if( x.style.display!='none' ){ +x.style.display = 'none'; +b.innerHTML='show'; +}else{ +x.style.display = ''; +b.innerHTML='hide'; +} +return false; +} +var antiRobot = 0; +function antiRobotGo(){ +if( antiRobot!=3 ) return; +antiRobot = 7; +var j = document.getElementById("mtimelink"); +if(j && j.hasAttribute("data-href")) j.href=j.getAttribute("data-href"); +} +function antiRobotDefense(){ +document.body.onmousedown=function(){ +antiRobot |= 2; +antiRobotGo(); +document.body.onmousedown=null; +} +document.body.onmousemove=function(){ +antiRobot |= 2; +antiRobotGo(); +document.body.onmousemove=null; +} +setTimeout(function(){ +antiRobot |= 1; +antiRobotGo(); +}, 100) +antiRobotGo(); +} +antiRobotDefense(); +</script> +<div class=fancy> +<div class=nosearch> +<div class="fancy_title"> +VACUUM +</div> +</div> + + + + +<h1 id="syntax"><span>1. </span>Syntax</h1> +<p><b><a href="syntax/vacuum-stmt.html">vacuum-stmt:</a></b> +<button id='x0fc49388' onclick='hideorshow("x0fc49388","x74e14988")'>hide</button></p> + <div id='x74e14988' class='imgcontainer'> + <div style="max-width:599px"><svg xmlns='http://www.w3.org/2000/svg' class="pikchr" viewBox="0 0 599.669 64.8"> +<circle cx="5" cy="17" r="3.6" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<polygon points="32,17 20,21 20,12" style="fill:rgb(0,0,0)"/> +<path d="M9,17L26,17" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<path d="M47,32L108,32A15 15 0 0 0 123 17A15 15 0 0 0 108 2L47,2A15 15 0 0 0 32 17A15 15 0 0 0 47 32Z" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<text x="78" y="17" text-anchor="middle" fill="rgb(0,0,0)" dominant-baseline="central">VACUUM</text> +<polygon points="165,47 153,51 153,43" style="fill:rgb(0,0,0)"/> +<path d="M123,17 L 131,17 Q 138,17 138,32 L 138,32 Q 138,47 149,47 L 159,47" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<path d="M180,62L276,62A15 15 0 0 0 292 47L292,47A15 15 0 0 0 276 32L180,32A15 15 0 0 0 165 47L165,47A15 15 0 0 0 180 62Z" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<text x="228" y="47" text-anchor="middle" fill="rgb(0,0,0)" dominant-baseline="central">schema-name</text> +<polygon points="333,17 322,21 322,12" style="fill:rgb(0,0,0)"/> +<path d="M292,47 L 299,47 Q 307,47 307,32 L 307,32 Q 307,17 317,17 L 327,17" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<polygon points="375,47 363,51 363,43" style="fill:rgb(0,0,0)"/> +<path d="M333,17 L 341,17 Q 348,17 348,32 L 348,32 Q 348,47 359,47 L 369,47" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<path d="M390,62L417,62A15 15 0 0 0 432 47L432,47A15 15 0 0 0 417 32L390,32A15 15 0 0 0 375 47L375,47A15 15 0 0 0 390 62Z" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<text x="403" y="47" text-anchor="middle" fill="rgb(0,0,0)" dominant-baseline="central">INTO</text> +<polygon points="455,47 443,51 443,43" style="fill:rgb(0,0,0)"/> +<path d="M432,47L449,47" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<path d="M470,62L524,62A15 15 0 0 0 539 47L539,47A15 15 0 0 0 524 32L470,32A15 15 0 0 0 455 47L455,47A15 15 0 0 0 470 62Z" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<text x="497" y="47" text-anchor="middle" fill="rgb(0,0,0)" dominant-baseline="central">filename</text> +<polygon points="590,17 578,21 578,12" style="fill:rgb(0,0,0)"/> +<path d="M539,47 L 546,47 Q 554,47 554,32 L 554,32 Q 554,17 569,17 L 569,17 L 584,17" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<circle cx="593" cy="17" r="3.6" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<path d="M123,17L578,17" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +</svg> +</div> +</div> + + +<h1 id="description"><span>2. </span>Description</h1> +<p> + The VACUUM command rebuilds the database file, repacking it into a minimal + amount of disk space. There are several + reasons an application might do this: + +</p><ul> + <li> <p> Unless SQLite is running in "auto_vacuum=FULL" mode, when a large + amount of data is deleted from the database file it leaves behind empty + space, or "free" database pages. This means the database file might + be larger than strictly necessary. Running VACUUM to rebuild the + database reclaims this space and reduces the size of the database file. + + </p></li><li> <p> Frequent inserts, updates, and deletes can cause the database file + to become fragmented - where data for a single table or index is scattered + around the database file. Running VACUUM ensures that each table and + index is largely stored contiguously within the database file. In some + cases, VACUUM may also reduce the number of partially filled pages in + the database, reducing the size of the database file further. + + </p></li><li> <p> When content is deleted from an SQLite database, the content is not + usually erased but rather the space used to hold the content is marked as + being available for reuse. This can allow deleted content to be recovered + by a hacker or by forensic analysis. Running VACUUM will clean the database + of all traces of deleted content, thus preventing an adversary from recovering + deleted content. Using VACUUM in this way is an alternative to setting + <a href="pragma.html#pragma_secure_delete">PRAGMA secure_delete=ON</a>. + + </p></li><li> <p> Normally, the database <a href="pragma.html#pragma_page_size">page_size</a> and whether or not the database + supports <a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a> must be configured before the database file is + actually created. However, when not in <a href="wal.html">write-ahead log</a> mode, the + <a href="pragma.html#pragma_page_size">page_size</a> and/or <a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a> properties of an existing database may be + changed by using the <a href="pragma.html#pragma_page_size">page_size</a> and/or + <a href="pragma.html#pragma_auto_vacuum">pragma auto_vacuum</a> pragmas and then immediately VACUUMing + the database. When in <a href="wal.html">write-ahead log</a> mode, only the <a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a> + support property can be changed using VACUUM. +</p></li></ul> + +<p>By default, VACUUM operates on the main database. +<a href="lang_attach.html">Attached databases</a> can be vacuumed by appending the appropriate +<span class='yyterm'>schema-name</span> to the VACUUM statement. + +</p><p><b>Compatibility Warning:</b> The ability to vacuum attached databases was +added in <a href="releaselog/3_15_0.html">version 3.15.0</a> (2016-10-14). Prior to that, a +<span class='yyterm'>schema-name</span> added to the +VACUUM statement would be silently ignored and the "main" schema would be +vacuumed.</p> + +<a name="vacuuminto"></a> + +<h2 id="vacuum_with_an_into_clause"><span>2.1. </span>VACUUM with an INTO clause</h2> + +<p>If the INTO clause is included, then the original database file is +unchanged and a new database is created in a file named by the +argument to the INTO clause. +The argument is a scalar <a href="lang_expr.html">expression</a>, such as a text literal. +The new database will contain the same +logical content as the original database, fully vacuumed. + +</p><p> +The VACUUM command with an INTO clause is an alternative to the +<a href="backup.html">backup API</a> for generating backup copies of a live database. +The advantage of using VACUUM INTO is that the resulting backup +database is minimal in size and hence the amount of filesystem +I/O may be reduced. Also, all deleted content is purged from the +backup, leaving behind no forensic traces. On the other hand, +the <a href="backup.html">backup API</a> uses fewer CPU cycles and can be executed +incrementally. + +</p><p> +The filename in the INTO clause can be an arbitrary SQL expression +that evaluates to a string. +The file named by the INTO clause must not previously exist, or +else it must be an empty file, or the VACUUM INTO command will +fail with an error. + +</p><p> +The argument to INTO can be a <a href="uri.html">URI filename</a> if URI filenames +are enabled. +URL filenames are enabled if any of the following are true: +</p><ul> +<li> The SQLite library was compiled with <a href="compile.html#use_uri">-DSQLITE_USE_URI=1</a>. +</li><li> The <a href="c3ref/config.html">sqlite3_config</a>(<a href="c3ref/c_config_covering_index_scan.html#sqliteconfiguri">SQLITE_CONFIG_URI</a>,1) interfaces was + invoked at start-time. +</li><li> The <a href="c3ref/sqlite3.html">database connection</a> that is running the VACUUM INTO + statement was originally opened using the + <a href="c3ref/c_open_autoproxy.html">SQLITE_OPEN_URI</a> flag. +</li></ul> + +<p> +The VACUUM INTO command is transactional in the sense that +the generated output database is a consistent snapshot of the +original database. However, if the VACUUM INTO command is +interrupted by an unplanned shutdown or power loss, then +the generated output database might be incomplete and corrupt. +Also, SQLite does not invoke fsync() or FlushFileBuffers() +on the generated database to ensure that it has reached +non-volatile storage before completing. + + +<a name="howvacuumworks"></a> + +</p><h1 id="how_vacuum_works"><span>3. </span>How VACUUM works</h1> + +<p>The VACUUM command works by copying the contents of the database into +a temporary database file and then overwriting the original with the +contents of the temporary file. When overwriting the original, a rollback +journal or <a href="wal.html">write-ahead log</a> WAL file is used just as it would be for any +other database transaction. This means that when VACUUMing a database, +as much as twice the size of the original database file is required in free +disk space. + +</p><p>The VACUUM INTO command works the same way except that it uses the file +named on the INTO clause in place of the temporary database and omits the +step of copying the vacuumed database back over top of the original database. + +</p><p>The VACUUM command may change the <a href="lang_createtable.html#rowid">ROWIDs</a> of entries in any +tables that do not have an explicit <a href="lang_createtable.html#rowid">INTEGER PRIMARY KEY</a>. +</p> + +<p>A VACUUM will fail if there is an open transaction on the database +connection that is attempting to run the VACUUM. Unfinalized SQL +statements typically hold a read transaction open, so the VACUUM +might fail if there are unfinalized SQL statements on the same connection. +VACUUM (but not VACUUM INTO) is a write operation and so if another +database connection is holding a lock that prevents writes, then +the VACUUM will fail. + +</p><p>An alternative to using the VACUUM command to +reclaim space after data has been deleted is auto-vacuum mode, enabled using +the <a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a> pragma. When <a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a> is enabled for a database +free pages may be reclaimed after deleting data, causing the file to shrink, +without rebuilding the entire database using VACUUM. However, using +<a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a> can lead to extra database file fragmentation. And <a href="pragma.html#pragma_auto_vacuum">auto_vacuum</a> +does not compact partially filled pages of the database as VACUUM does. + +</p> +<p align="center"><small><i>This page last modified on <a href="https://sqlite.org/docsrc/honeypot" id="mtimelink" data-href="https://sqlite.org/docsrc/finfo/pages/lang_vacuum.in?m=d66fef5b60">2023-12-05 14:43:20</a> UTC </small></i></p> + |