<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>11.4. Indexes and ORDER BY</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="indexes-multicolumn.html" title="11.3. Multicolumn Indexes" /><link rel="next" href="indexes-bitmap-scans.html" title="11.5. Combining Multiple Indexes" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">11.4. Indexes and <code xmlns="http://www.w3.org/1999/xhtml" class="literal">ORDER BY</code></th></tr><tr><td width="10%" align="left"><a accesskey="p" href="indexes-multicolumn.html" title="11.3. Multicolumn Indexes">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="indexes.html" title="Chapter 11. Indexes">Up</a></td><th width="60%" align="center">Chapter 11. Indexes</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="indexes-bitmap-scans.html" title="11.5. Combining Multiple Indexes">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="INDEXES-ORDERING"><div class="titlepage"><div><div><h2 class="title" style="clear: both">11.4. Indexes and <code class="literal">ORDER BY</code></h2></div></div></div><a id="id-1.5.10.7.2" class="indexterm"></a><p> In addition to simply finding the rows to be returned by a query, an index may be able to deliver them in a specific sorted order. This allows a query's <code class="literal">ORDER BY</code> specification to be honored without a separate sorting step. Of the index types currently supported by <span class="productname">PostgreSQL</span>, only B-tree can produce sorted output — the other index types return matching rows in an unspecified, implementation-dependent order. </p><p> The planner will consider satisfying an <code class="literal">ORDER BY</code> specification either by scanning an available index that matches the specification, or by scanning the table in physical order and doing an explicit sort. For a query that requires scanning a large fraction of the table, an explicit sort is likely to be faster than using an index because it requires less disk I/O due to following a sequential access pattern. Indexes are more useful when only a few rows need be fetched. An important special case is <code class="literal">ORDER BY</code> in combination with <code class="literal">LIMIT</code> <em class="replaceable"><code>n</code></em>: an explicit sort will have to process all the data to identify the first <em class="replaceable"><code>n</code></em> rows, but if there is an index matching the <code class="literal">ORDER BY</code>, the first <em class="replaceable"><code>n</code></em> rows can be retrieved directly, without scanning the remainder at all. </p><p> By default, B-tree indexes store their entries in ascending order with nulls last. This means that a forward scan of an index on column <code class="literal">x</code> produces output satisfying <code class="literal">ORDER BY x</code> (or more verbosely, <code class="literal">ORDER BY x ASC NULLS LAST</code>). The index can also be scanned backward, producing output satisfying <code class="literal">ORDER BY x DESC</code> (or more verbosely, <code class="literal">ORDER BY x DESC NULLS FIRST</code>, since <code class="literal">NULLS FIRST</code> is the default for <code class="literal">ORDER BY DESC</code>). </p><p> You can adjust the ordering of a B-tree index by including the options <code class="literal">ASC</code>, <code class="literal">DESC</code>, <code class="literal">NULLS FIRST</code>, and/or <code class="literal">NULLS LAST</code> when creating the index; for example: </p><pre class="programlisting"> CREATE INDEX test2_info_nulls_low ON test2 (info NULLS FIRST); CREATE INDEX test3_desc_index ON test3 (id DESC NULLS LAST); </pre><p> An index stored in ascending order with nulls first can satisfy either <code class="literal">ORDER BY x ASC NULLS FIRST</code> or <code class="literal">ORDER BY x DESC NULLS LAST</code> depending on which direction it is scanned in. </p><p> You might wonder why bother providing all four options, when two options together with the possibility of backward scan would cover all the variants of <code class="literal">ORDER BY</code>. In single-column indexes the options are indeed redundant, but in multicolumn indexes they can be useful. Consider a two-column index on <code class="literal">(x, y)</code>: this can satisfy <code class="literal">ORDER BY x, y</code> if we scan forward, or <code class="literal">ORDER BY x DESC, y DESC</code> if we scan backward. But it might be that the application frequently needs to use <code class="literal">ORDER BY x ASC, y DESC</code>. There is no way to get that ordering from a plain index, but it is possible if the index is defined as <code class="literal">(x ASC, y DESC)</code> or <code class="literal">(x DESC, y ASC)</code>. </p><p> Obviously, indexes with non-default sort orderings are a fairly specialized feature, but sometimes they can produce tremendous speedups for certain queries. Whether it's worth maintaining such an index depends on how often you use queries that require a special sort ordering. </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="indexes-multicolumn.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="indexes.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="indexes-bitmap-scans.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">11.3. Multicolumn Indexes </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 11.5. Combining Multiple Indexes</td></tr></table></div></body></html>
