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diff --git a/www/lang_explain.html b/www/lang_explain.html new file mode 100644 index 0000000..be2e055 --- /dev/null +++ b/www/lang_explain.html @@ -0,0 +1,398 @@ +<!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>EXPLAIN</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. 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+antiRobotGo(); +}, 100) +antiRobotGo(); +} +antiRobotDefense(); +</script> +<div class=fancy> +<div class=nosearch> +<div class="fancy_title"> +EXPLAIN +</div> +</div> + + + + +<h1 id="syntax"><span>1. </span>Syntax</h1> +<p><b><a href="syntax/sql-stmt.html">sql-stmt:</a></b></p><div class='imgcontainer'> + <div style="max-width:716px"><svg xmlns='http://www.w3.org/2000/svg' class="pikchr" viewBox="0 0 716.88 1017.36"> +<circle cx="5" cy="17" r="3.6" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<polygon points="50,55 39,59 39,50" style="fill:rgb(0,0,0)"/> +<path d="M9,17 L 16,17 Q 24,17 24,32 L 24,40 Q 24,55 34,55 L 45,55" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<path d="M66,70L125,70A15 15 0 0 0 140 55L140,55A15 15 0 0 0 125 39L66,39A15 15 0 0 0 50 55L50,55A15 15 0 0 0 66 70Z" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<text x="95" y="55" text-anchor="middle" fill="rgb(0,0,0)" dominant-baseline="central">EXPLAIN</text> +<polygon 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style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<path d="M666,962 L 673,962 Q 681,962 681,954 L 681,947" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<path d="M410,1015L529,1015L529,984L410,984Z" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<text x="469" y="1000" text-anchor="middle" fill="rgb(0,0,0)" dominant-baseline="central">vacuum-stmt</text> +<polygon points="410,1000 398,1004 398,995" style="fill:rgb(0,0,0)"/> +<path d="M368,17 L 376,17 Q 383,17 383,32 L 383,985 Q 383,1000 393,1000 L 404,1000" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<polygon points="666,1000 654,1004 654,995" style="fill:rgb(0,0,0)"/> +<path d="M529,1000L660,1000" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<path d="M666,1000 L 673,1000 Q 681,1000 681,985 L 681,32 Q 681,17 688,17 L 696,17" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +</svg> +</div> + </div> + + +<h1 id="description"><span>2. </span>Description</h1> + +<p>An SQL statement can be preceded by the keyword "EXPLAIN" or +by the phrase "EXPLAIN QUERY PLAN". Either modification causes the +SQL statement to behave as a query and to return information about +how the SQL statement would have operated if the EXPLAIN keyword or +phrase had been omitted.</p> + +<p>The output from EXPLAIN and EXPLAIN QUERY PLAN is intended for +interactive analysis and troubleshooting only. The details of the +output format are subject to change from one release of SQLite to the next. +Applications should not use EXPLAIN or EXPLAIN QUERY PLAN since +their exact behavior is variable and only partially documented.</p> + +<p>When the EXPLAIN keyword appears by itself it causes the statement +to behave as a query that returns the sequence of +<a href="opcode.html">virtual machine instructions</a> it would have used to execute the command had +the EXPLAIN keyword not been present. When the EXPLAIN QUERY PLAN phrase +appears, the statement returns high-level information regarding the query +plan that would have been used. + +</p><p>The EXPLAIN QUERY PLAN command is described in +<a href="eqp.html">more detail here</a>. + +</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> + +<p>The EXPLAIN and EXPLAIN QUERY PLAN prefixes affect the behavior of +running a <a href="c3ref/stmt.html">prepared statement</a> using <a href="c3ref/step.html">sqlite3_step()</a>. The process of +generating a new prepared statement using <a href="c3ref/prepare.html">sqlite3_prepare()</a> or similar +is (mostly) unaffected by EXPLAIN. (The exception to the previous sentence +is that some special opcodes used by EXPLAIN QUERY PLAN are omitted when +building an EXPLAIN QUERY PLAN prepared statement, as a performance +optimization.) + +</p><p>This means that actions that occur during sqlite3_prepare() are +unaffected by EXPLAIN. + +</p><ul> +<li><p> +Some <a href="pragma.html#syntax">PRAGMA</a> statements do their work during sqlite3_prepare() rather +than during sqlite3_step(). Those PRAGMA statements are unaffected +by EXPLAIN. They operate the same with or without the EXPLAIN prefix. +The set of PRAGMA statements that are unaffected by EXPLAIN can vary +from one release to the next. Some PRAGMA statements operate during +sqlite3_prepare() depending on their arguments. For consistent +results, avoid using EXPLAIN on PRAGMA statements. + +</p></li><li><p> +The <a href="c3ref/set_authorizer.html">authorizer callback</a> is invoked regardless of the presence of +EXPLAIN or EXPLAIN QUERY PLAN. +</p></li></ul> +<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_explain.in?m=7122ecf5f68ffd8ff">2020-02-10 20:33:26</a> UTC </small></i></p> + |