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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: INDEXED BY</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 align="center">SQL As Understood By SQLite</h2></a><h1>INDEXED BY</h1><p>The INDEXED BY phrase is a SQL extension found only in SQLite which canbe used to verify that the correct indices are being used on a <a href="lang_delete.html">DELETE</a>,<a href="lang_select.html">SELECT</a>, or <a href="lang_update.html">UPDATE</a> statement.The INDEXED BY phrase always follows the name of a table that SQLite willbe reading. The INDEXED BY phrase can be seen in the following syntaxdiagrams:</p><h4><a href="syntaxdiagrams.html#qualified-table-name">qualified-table-name:</a></h4><blockquote> <img src="images/syntax/qualified-table-name.gif"></img> </blockquote><h4><a href="syntaxdiagrams.html#single-source">single-source:</a></h4><blockquote> <img src="images/syntax/single-source.gif"></img> </blockquote><p>The "INDEXED BY index-name" clause specifies that the named indexmust be used in order to look up values on the preceeding table.If index-name does not exist or cannot be used for the query, thenthe preparation of the SQL statement fails.The "NOT INDEXED" clause specifies that no index shall be used whenaccessing the preceeding table, including implied indices create byUNIQUE and PRIMARY KEY constraints. However, the INTEGER PRIMARY KEYcan still be used to look up entries even when "NOT INDEXED" is specified.</p><p>Some SQL database engines provide non-standard "hint" mechanisms whichcan be used to give the query optimizer clues about what indices it shoulduse for a particular statement. The INDEX BY clause of SQLite is <em>not</em> a hinting mechanism and it should not be used as such.The INDEXED BY clause does not give the optimizer hints about which indexto use; it gives the optimizer a requirement of which index to use.If the query optimizer is unable to use the index specified by theINDEX BY clause, then the query will fail with an error.</p><p>The INDEXED BY clause is <em>not</em> intended for use in tuningthe preformance of a query. The intent of the INDEXED BY clause isto raise a run-time error if a schema change, such as dropping orcreating an index, causes the query plan for a time-sensitive queryto change. The INDEXED BY clause is designed to help detectundesirable query plan changes during regression testing.Developers are admonished to omit all use of INDEXED BY duringapplication design, implementation, testing, and tuning. IfINDEXED BY is to be used at all, it should be inserted at the veryend of the development process when "locking down" a design.</p><DIV class="pdf_section"><hr><small><i>This page last modified 2009/01/02 16:47:13 UTC</i></small></div></body></html>⌨️ 快捷键说明
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