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diff --git a/www/lang_analyze.html b/www/lang_analyze.html new file mode 100644 index 0000000..7b90e97 --- /dev/null +++ b/www/lang_analyze.html @@ -0,0 +1,350 @@ +<!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>ANALYZE</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"> +ANALYZE +</div> +</div> + + + + +<h1 id="overview"><span>1. </span>Overview</h1> + +<p><b><a href="syntax/analyze-stmt.html">analyze-stmt:</a></b> +<button id='x1333' onclick='hideorshow("x1333","x1334")'>hide</button></p> + <div id='x1334' class='imgcontainer'> + <div style="max-width:654px"><svg xmlns='http://www.w3.org/2000/svg' class="pikchr" viewBox="0 0 654.557 140.4"> +<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="M47,32L111,32A15 15 0 0 0 127 17A15 15 0 0 0 111 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="79" y="17" text-anchor="middle" fill="rgb(0,0,0)" dominant-baseline="central">ANALYZE</text> +<polygon points="150,17 138,21 138,12" style="fill:rgb(0,0,0)"/> +<path d="M127,17L144,17" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<polygon points="188,123 176,127 176,118" style="fill:rgb(0,0,0)"/> +<path d="M150,17 L 157,17 Q 165,17 165,32 L 165,108 Q 165,123 173,123 L 182,123" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<path d="M203,138L299,138A15 15 0 0 0 314 123A15 15 0 0 0 299 108L203,108A15 15 0 0 0 188 123A15 15 0 0 0 203 138Z" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<text x="251" y="123" text-anchor="middle" fill="rgb(0,0,0)" dominant-baseline="central">schema-name</text> +<polygon points="337,123 326,127 326,118" style="fill:rgb(0,0,0)"/> +<path d="M314,123L332,123" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<path d="M352,138A15 15 0 0 0 368 123A15 15 0 0 0 352 108A15 15 0 0 0 337 123A15 15 0 0 0 352 138Z" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<text x="352" y="123" text-anchor="middle" font-weight="bold" fill="rgb(0,0,0)" dominant-baseline="central">.</text> +<polygon points="391,123 379,127 379,118" style="fill:rgb(0,0,0)"/> +<path d="M368,123L385,123" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<path d="M406,138L556,138A15 15 0 0 0 571 123A15 15 0 0 0 556 108L406,108A15 15 0 0 0 391 123A15 15 0 0 0 406 138Z" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<text x="481" y="123" text-anchor="middle" fill="rgb(0,0,0)" dominant-baseline="central">table-or-index-name</text> +<polygon points="594,123 582,127 582,118" style="fill:rgb(0,0,0)"/> +<path d="M571,123L588,123" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<polygon points="645,17 633,21 633,12" style="fill:rgb(0,0,0)"/> +<path d="M594,123 L 601,123 Q 609,123 609,108 L 609,32 Q 609,17 624,17 L 624,17 L 639,17" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<circle cx="648" cy="17" r="3.6" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<polygon points="397,17 386,21 386,12" style="fill:rgb(0,0,0)"/> +<path d="M150,17L391,17" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<path d="M397,17L633,17" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<polygon points="188,47 176,51 176,43" style="fill:rgb(0,0,0)"/> +<path d="M150,17 L 157,17 Q 165,17 165,32 L 165,32 Q 165,47 173,47 L 182,47" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<path d="M203,62L299,62A15 15 0 0 0 314 47L314,47A15 15 0 0 0 299 32L203,32A15 15 0 0 0 188 47L188,47A15 15 0 0 0 203 62Z" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<text x="251" y="47" text-anchor="middle" fill="rgb(0,0,0)" dominant-baseline="central">schema-name</text> +<polygon points="594,47 582,51 582,43" style="fill:rgb(0,0,0)"/> +<path d="M314,47L588,47" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<path d="M594,47 L 601,47 Q 609,47 609,40 L 609,32" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<polygon points="188,85 176,89 176,81" style="fill:rgb(0,0,0)"/> +<path d="M150,17 L 157,17 Q 165,17 165,32 L 165,70 Q 165,85 173,85 L 182,85" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<path d="M203,100L353,100A15 15 0 0 0 368 85A15 15 0 0 0 353 70L203,70A15 15 0 0 0 188 85A15 15 0 0 0 203 100Z" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<text x="278" y="85" text-anchor="middle" fill="rgb(0,0,0)" dominant-baseline="central">index-or-table-name</text> +<polygon points="594,85 582,89 582,81" style="fill:rgb(0,0,0)"/> +<path d="M368,85L588,85" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +<path d="M594,85 L 601,85 Q 609,85 609,77 L 609,70" style="fill:none;stroke-width:2.16;stroke:rgb(0,0,0);" /> +</svg> +</div> +</div> + + +<p> The ANALYZE command gathers statistics about tables and +indices and stores the collected information +in <a href="fileformat2.html#intschema">internal tables</a> of the database where the query optimizer can +access the information and use it to help make better query planning choices. +If no arguments are given, all attached databases are +analyzed. If a schema name is given as the argument, then all tables +and indices in that one database are analyzed. +If the argument is a table name, then only that table and the +indices associated with that table are analyzed. If the argument +is an index name, then only that one index is analyzed.</p> + +<a name="req"></a> + +<h2 id="recommended_usage_pattern"><span>1.1. </span>Recommended usage pattern</h2> + +<p>Applications with long-lived databases that use complex +queries should consider running the following commands just prior +to closing each database connection: + +</p><div class="codeblock"><pre>PRAGMA analysis_limit=400; +PRAGMA optimize; +</pre></div> + +<p>The <a href="pragma.html#pragma_optimize">optimize pragma</a> is usually a no-op but it will occasionally +run ANALYZE if it seems like doing so will be useful to the query planner. +The <a href="pragma.html#pragma_analysis_limit">analysis_limit pragma</a> limits the scope of any ANALYZE command that +the <a href="pragma.html#pragma_optimize">optimize pragma</a> runs so that it does not consume too many CPU cycles. +The constant "400" can be adjusted as needed. Values between 100 and +1000 work well for most applications. + +</p><h1 id="details"><span>2. </span>Details</h1> + +<p> The default implementation stores all statistics in a single +table named "<a href="fileformat2.html#stat1tab">sqlite_stat1</a>". + If SQLite is compiled with the +<a href="compile.html#enable_stat4">SQLITE_ENABLE_STAT4</a> option, then additional histogram data is +collected and stored in <a href="fileformat2.html#stat4tab">sqlite_stat4</a>. +Older versions of SQLite would make use of the <a href="fileformat2.html#stat2tab">sqlite_stat2</a> table +or <a href="fileformat2.html#stat3tab">sqlite_stat3</a> table +when compiled with <a href="compile.html#enable_stat2">SQLITE_ENABLE_STAT2</a> or <a href="compile.html#enable_stat3">SQLITE_ENABLE_STAT3</a>, +but all recent versions of +SQLite ignore the sqlite_stat2 and sqlite_stat3 tables. +Future enhancements may create +additional <a href="fileformat2.html#intschema">internal tables</a> with the same name pattern except with +final digit larger than "4". +All of these tables are collectively referred to as "statistics tables". +</p> + +<p> The content of the statistics tables can be queried using <a href="lang_select.html">SELECT</a> +and can be changed using the <a href="lang_delete.html">DELETE</a>, <a href="lang_insert.html">INSERT</a>, and <a href="lang_update.html">UPDATE</a> commands. +The <a href="lang_droptable.html">DROP TABLE</a> command works on statistics tables +as of SQLite version 3.7.9. (2011-11-01) +The <a href="lang_altertable.html">ALTER TABLE</a> command does not work on statistics tables. +Appropriate care should be used when changing the content of the statistics +tables as invalid content can cause SQLite to select inefficient +query plans. Generally speaking, one should not modify the content of +the statistics tables by any mechanism other than invoking the +ANALYZE command. +See "<a href="optoverview.html#manctrl">Manual Control Of Query Plans Using SQLITE_STAT Tables</a>" for +further information.</p> + +<p> Statistics gathered by ANALYZE are not automatically updated as +the content of the database changes. If the content of the database +changes significantly, or if the database schema changes, then one should +consider rerunning the ANALYZE command in order to update the statistics.</p> + +<p> The query planner loads the content of the statistics tables +into memory when the schema is read. Hence, when an application +changes the statistics tables directly, SQLite will not immediately +notice the changes. An application +can force the query planner to reread the statistics tables by running +<b>ANALYZE sqlite_schema</b>. </p> + +<a name="autoanalyze"></a> + +<h1 id="automatically_running_analyze"><span>3. </span>Automatically Running ANALYZE</h1> + +<p>The <a href="pragma.html#pragma_optimize">PRAGMA optimize</a> command will automatically run ANALYZE on individual +tables on an as-needed basis. The recommended practice is for applications +to invoke the <a href="pragma.html#pragma_optimize">PRAGMA optimize</a> statement just before closing each database +connection.</p> + +<p>Each SQLite <a href="c3ref/sqlite3.html">database connection</a> records cases when the query planner would +benefit from having accurate results of ANALYZE at hand. These records +are held in memory and accumulate over the life of a database connection. +The <a href="pragma.html#pragma_optimize">PRAGMA optimize</a> command looks at those records and runs ANALYZE on only +those tables for which new or updated ANALYZE data seems likely to be useful. +In most cases <a href="pragma.html#pragma_optimize">PRAGMA optimize</a> will not run ANALYZE, but it will occasionally +do so either for tables that have never before been analyzed, or for tables +that have grown significantly since they were last analyzed.</p> + +<p>Since the actions of <a href="pragma.html#pragma_optimize">PRAGMA optimize</a> are determined to some extent by +prior queries that have been evaluated on the same database connection, it +is recommended that <a href="pragma.html#pragma_optimize">PRAGMA optimize</a> be deferred until the database connection +is closing and has thus had an opportunity to accumulate as much usage information +as possible. It is also reasonable to set a timer to run <a href="pragma.html#pragma_optimize">PRAGMA optimize</a> +every few hours, or every few days, for database connections that stay open +for a long time.</p> + +<p>Applications that desire more control can run <a href="pragma.html#pragma_optimize">PRAGMA optimize(0x03)</a> to +obtain a list of ANALYZE commands that SQLite thinks are appropriate to run, +but without actually running those commands. If the returned set is +non-empty, the application can then make a decision about whether or not +to run the suggested ANALYZE commands, perhaps after prompting the user +for guidance.</p> + +<p>The <a href="pragma.html#pragma_optimize">PRAGMA optimize</a> command was first introduced with +SQLite 3.18.0 (2017-03-28) and is a no-op for all prior releases +of SQLite.</p> + +<a name="approx"></a> + +<h1 id="approximate_analyze_for_large_databases"><span>4. </span>Approximate ANALYZE For Large Databases</h1> + +<p>By default, ANALYZE does a full scan of every index. This can be slow for +large databases. So beginning with SQLite version 3.32.0 (2020-05-22), the +<a href="pragma.html#pragma_analysis_limit">PRAGMA analysis_limit</a> command can be used to limit the amount of +scanning performed by ANALYZE, and thus help ANALYZE to run faster, +even on very large database files. We call this running an +"approximate ANALYZE". + +</p><p>The recommended usage pattern for the <a href="pragma.html#pragma_analysis_limit">analysis_limit</a> pragma is +like this: + +</p><div class="codeblock"><pre>PRAGMA analysis_limit=1000; +</pre></div> + +<p>This pragma tells the ANALYZE command to start a full scan +of the index as it normally would. But when the number of rows visited +reaches 1000 (or whatever other limit is specified by the pragma), the +ANALYZE command will begin taking actions to stop the scan. If +the left-most column of the index has changed at least once during the +previous 1000 steps, then the analysis stops immediately. But if the +left-most column has always been the same, then ANALYZE skips ahead to +the first entry with a different left-most column and reads an additional +1000 rows before terminating. + +</p><p>The details of the effects of the analysis limit described in the previous +paragraph are subject to change in future versions of SQLite. But the +core idea will remain the same. An analysis limit of N will strive to +limit the number of rows visited in each index to approximately N. + +</p><p>Values of N between 100 and 1000 are recommended. +Or, to disable the analysis limit, causing ANALYZE to do a +complete scan of each index, set the analysis limit to 0. The default +value for the analysis limit is 0 for backwards compatibility. + +</p><p>The values placed in the sqlite_stat1 table by an approximate ANALYZE +are not exactly the same as what would be computed by an unrestricted +analysis. But they are usually close enough. The index statistics in +the sqlite_stat1 table are approximations in any case, so the fact that +the results of an approximate ANALYZE are slightly different from +a traditional full scan ANALYZE has little practical impact. It is +possible to construct a pathological case where an approximate ANALYZE +is noticeably inferior to a full-scan ANALYZE, but such cases rare in +real-world problems. + +</p><p>A good rule of thumb seems to be to always set "PRAGMA analysis_limit=N" +for N between 100 and 1000 prior to running either "ANALYZE" or +"<a href="pragma.html#pragma_optimize">PRAGMA optimize</a>". The results are not quite as precise, but they +are precise enough, and the fact that the results are computed so much +faster means that developers are more likely to compute them. An +approximate ANALYZE is better than not running ANALYZE at all. + +</p><h2 id="limitations_of_approximate_analyze"><span>4.1. </span>Limitations of approximate ANALYZE</h2> + +<p>The content in the sqlite_stat4 table cannot be computed with +anything less than a full scan. Hence, if a non-zero analysis limit +is specified, the sqlite_stat4 table is not computed. +</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_analyze.in?m=85eb0dec08045a663">2020-06-18 21:18:56</a> UTC </small></i></p> + |