162 lines
5.7 KiB
HTML
162 lines
5.7 KiB
HTML
<!DOCTYPE html>
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<html><head>
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<meta name="viewport" content="width=device-width, initial-scale=1.0">
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<meta http-equiv="content-type" content="text/html; charset=UTF-8">
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<link href="sqlite.css" rel="stylesheet">
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<title>EXPLAIN</title>
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<!-- path= -->
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</head>
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<body>
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<div class=nosearch>
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<a href="index.html">
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<img class="logo" src="images/sqlite370_banner.gif" alt="SQLite" border="0">
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</a>
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<div><!-- IE hack to prevent disappearing logo --></div>
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<div class="tagline desktoponly">
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Small. Fast. Reliable.<br>Choose any three.
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</div>
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<div class="menu mainmenu">
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<ul>
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<li><a href="index.html">Home</a>
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<li class='mobileonly'><a href="javascript:void(0)" onclick='toggle_div("submenu")'>Menu</a>
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<li class='wideonly'><a href='about.html'>About</a>
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<li class='desktoponly'><a href="docs.html">Documentation</a>
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<li class='desktoponly'><a href="download.html">Download</a>
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<li class='wideonly'><a href='copyright.html'>License</a>
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<li class='desktoponly'><a href="support.html">Support</a>
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<li class='desktoponly'><a href="prosupport.html">Purchase</a>
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<li class='search' id='search_menubutton'>
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<a href="javascript:void(0)" onclick='toggle_search()'>Search</a>
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</ul>
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</div>
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<div class="menu submenu" id="submenu">
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<ul>
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<li><a href='about.html'>About</a>
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<li><a href='docs.html'>Documentation</a>
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<li><a href='download.html'>Download</a>
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<li><a href='support.html'>Support</a>
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<li><a href='prosupport.html'>Purchase</a>
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</ul>
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</div>
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<div class="searchmenu" id="searchmenu">
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<form method="GET" action="search">
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<select name="s" id="searchtype">
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<option value="d">Search Documentation</option>
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<option value="c">Search Changelog</option>
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</select>
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<input type="text" name="q" id="searchbox" value="">
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<input type="submit" value="Go">
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</form>
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</div>
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</div>
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<script>
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function toggle_div(nm) {
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var w = document.getElementById(nm);
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if( w.style.display=="block" ){
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w.style.display = "none";
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}else{
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w.style.display = "block";
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}
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}
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function toggle_search() {
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var w = document.getElementById("searchmenu");
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if( w.style.display=="block" ){
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w.style.display = "none";
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} else {
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w.style.display = "block";
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setTimeout(function(){
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document.getElementById("searchbox").focus()
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}, 30);
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}
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}
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function div_off(nm){document.getElementById(nm).style.display="none";}
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window.onbeforeunload = function(e){div_off("submenu");}
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/* Disable the Search feature if we are not operating from CGI, since */
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/* Search is accomplished using CGI and will not work without it. */
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if( !location.origin || !location.origin.match || !location.origin.match(/http/) ){
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document.getElementById("search_menubutton").style.display = "none";
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}
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/* Used by the Hide/Show button beside syntax diagrams, to toggle the */
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function hideorshow(btn,obj){
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var x = document.getElementById(obj);
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var b = document.getElementById(btn);
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if( x.style.display!='none' ){
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x.style.display = 'none';
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b.innerHTML='show';
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}else{
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x.style.display = '';
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b.innerHTML='hide';
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}
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return false;
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}
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</script>
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</div>
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<div class=fancy>
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<div class=nosearch>
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<div class="fancy_title">
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EXPLAIN
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</div>
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</div>
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<h1 id="syntax"><span>1. </span>Syntax</h1>
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<p><b><a href="syntax/sql-stmt.html">sql-stmt:</a></b></p><div class='imgcontainer'>
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<img alt="syntax diagram sql-stmt" src="images/syntax/sql-stmt.gif"></img>
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</div>
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<h1 id="description"><span>2. </span>Description</h1>
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<p>An SQL statement can be preceded by the keyword "EXPLAIN" or
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by the phrase "EXPLAIN QUERY PLAN". Either modification causes the
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SQL statement to behave as a query and to return information about
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how the SQL statement would have operated if the EXPLAIN keyword or
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phrase had been omitted.</p>
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<p>The output from EXPLAIN and EXPLAIN QUERY PLAN is intended for
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interactive analysis and troubleshooting only. The details of the
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output format are subject to change from one release of SQLite to the next.
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Applications should not use EXPLAIN or EXPLAIN QUERY PLAN since
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their exact behavior is variable and only partially documented.</p>
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<p>When the EXPLAIN keyword appears by itself it causes the statement
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to behave as a query that returns the sequence of
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<a href="opcode.html">virtual machine instructions</a> it would have used to execute the command had
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the EXPLAIN keyword not been present. When the EXPLAIN QUERY PLAN phrase
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appears, the statement returns high-level information regarding the query
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plan that would have been used.
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</p><p>The EXPLAIN QUERY PLAN command is described in
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<a href="eqp.html">more detail here</a>.
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</p><h2 id="explain_operates_at_run_time_not_at_prepare_time"><span>2.1. </span>EXPLAIN operates at run-time, not at prepare-time</h2>
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<p>The EXPLAIN and EXPLAIN QUERY PLAN prefixes affect the behavior of
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running a <a href="c3ref/stmt.html">prepared statement</a> using <a href="c3ref/step.html">sqlite3_step()</a>. The process of
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generating a new prepared statement using <a href="c3ref/prepare.html">sqlite3_prepare()</a> or similar
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is (mostly) unaffected by EXPLAIN. (The exception to the previous sentence
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is that some special opcodes used by EXPLAIN QUERY PLAN are omitted when
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building an EXPLAIN QUERY PLAN prepared statement, as a performance
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optimization.)
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</p><p>This means that actions that occur during sqlite3_prepare() are
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unaffected by EXPLAIN.
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</p><ul>
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<li><p>
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Some <a href="pragma.html#syntax">PRAGMA</a> statements do their work during sqlite3_prepare() rather
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than during sqlite3_step(). Those PRAGMA statements are unaffected
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by EXPLAIN. They operate the same with or without the EXPLAIN prefix.
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The set of PRAGMA statements that are unaffected by EXPLAIN can vary
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from one release to the next. Some PRAGMA statements operate during
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sqlite3_prepare() depending on their arguments. For consistent
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results, avoid using EXPLAIN on PRAGMA statements.
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</p></li><li><p>
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The <a href="c3ref/set_authorizer.html">authorizer callback</a> is invoked regardless of the presence of
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EXPLAIN or EXPLAIN QUERY PLAN.
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</p></li></ul>
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