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1<?xml version="1.0" encoding="UTF-8" standalone="no"?>2<!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>64.6. Index Cost Estimation Functions</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="index-unique-checks.html" title="64.5. Index Uniqueness Checks" /><link rel="next" href="generic-wal.html" title="Chapter 65. Generic WAL Records" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">64.6. Index Cost Estimation Functions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="index-unique-checks.html" title="64.5. Index Uniqueness Checks">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="indexam.html" title="Chapter 64. Index Access Method Interface Definition">Up</a></td><th width="60%" align="center">Chapter 64. Index Access Method Interface Definition</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="generic-wal.html" title="Chapter 65. Generic WAL Records">Next</a></td></tr></table><hr /></div><div class="sect1" id="INDEX-COST-ESTIMATION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">64.6. Index Cost Estimation Functions <a href="#INDEX-COST-ESTIMATION" class="id_link">#</a></h2></div></div></div><p>3   The <code class="function">amcostestimate</code> function is given information describing4   a possible index scan, including lists of WHERE and ORDER BY clauses that5   have been determined to be usable with the index.  It must return estimates6   of the cost of accessing the index and the selectivity of the WHERE7   clauses (that is, the fraction of parent-table rows that will be8   retrieved during the index scan).  For simple cases, nearly all the9   work of the cost estimator can be done by calling standard routines10   in the optimizer; the point of having an <code class="function">amcostestimate</code> function is11   to allow index access methods to provide index-type-specific knowledge,12   in case it is possible to improve on the standard estimates.13  </p><p>14   Each <code class="function">amcostestimate</code> function must have the signature:15 16</p><pre class="programlisting">17void18amcostestimate (PlannerInfo *root,19                IndexPath *path,20                double loop_count,21                Cost *indexStartupCost,22                Cost *indexTotalCost,23                Selectivity *indexSelectivity,24                double *indexCorrelation,25                double *indexPages);26</pre><p>27 28   The first three parameters are inputs:29 30   </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="parameter"><code>root</code></em></span></dt><dd><p>31       The planner's information about the query being processed.32      </p></dd><dt><span class="term"><em class="parameter"><code>path</code></em></span></dt><dd><p>33       The index access path being considered.  All fields except cost and34       selectivity values are valid.35      </p></dd><dt><span class="term"><em class="parameter"><code>loop_count</code></em></span></dt><dd><p>36       The number of repetitions of the index scan that should be factored37       into the cost estimates.  This will typically be greater than one when38       considering a parameterized scan for use in the inside of a nestloop39       join.  Note that the cost estimates should still be for just one scan;40       a larger <em class="parameter"><code>loop_count</code></em> means that it may be appropriate41       to allow for some caching effects across multiple scans.42      </p></dd></dl></div><p>43  </p><p>44   The last five parameters are pass-by-reference outputs:45 46   </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="parameter"><code>*indexStartupCost</code></em></span></dt><dd><p>47       Set to cost of index start-up processing48      </p></dd><dt><span class="term"><em class="parameter"><code>*indexTotalCost</code></em></span></dt><dd><p>49       Set to total cost of index processing50      </p></dd><dt><span class="term"><em class="parameter"><code>*indexSelectivity</code></em></span></dt><dd><p>51       Set to index selectivity52      </p></dd><dt><span class="term"><em class="parameter"><code>*indexCorrelation</code></em></span></dt><dd><p>53       Set to correlation coefficient between index scan order and54       underlying table's order55      </p></dd><dt><span class="term"><em class="parameter"><code>*indexPages</code></em></span></dt><dd><p>56       Set to number of index leaf pages57      </p></dd></dl></div><p>58  </p><p>59   Note that cost estimate functions must be written in C, not in SQL or60   any available procedural language, because they must access internal61   data structures of the planner/optimizer.62  </p><p>63   The index access costs should be computed using the parameters used by64   <code class="filename">src/backend/optimizer/path/costsize.c</code>: a sequential65   disk block fetch has cost <code class="varname">seq_page_cost</code>, a nonsequential fetch66   has cost <code class="varname">random_page_cost</code>, and the cost of processing one index67   row should usually be taken as <code class="varname">cpu_index_tuple_cost</code>.  In68   addition, an appropriate multiple of <code class="varname">cpu_operator_cost</code> should69   be charged for any comparison operators invoked during index processing70   (especially evaluation of the indexquals themselves).71  </p><p>72   The access costs should include all disk and CPU costs associated with73   scanning the index itself, but <span class="emphasis"><em>not</em></span> the costs of retrieving or74   processing the parent-table rows that are identified by the index.75  </p><p>76   The <span class="quote">“<span class="quote">start-up cost</span>”</span> is the part of the total scan cost that77   must be expended before we can begin to fetch the first row.  For most78   indexes this can be taken as zero, but an index type with a high start-up79   cost might want to set it nonzero.80  </p><p>81   The <em class="parameter"><code>indexSelectivity</code></em> should be set to the estimated fraction of the parent82   table rows that will be retrieved during the index scan.  In the case83   of a lossy query, this will typically be higher than the fraction of84   rows that actually pass the given qual conditions.85  </p><p>86   The <em class="parameter"><code>indexCorrelation</code></em> should be set to the correlation (ranging between87   -1.0 and 1.0) between the index order and the table order.  This is used88   to adjust the estimate for the cost of fetching rows from the parent89   table.90  </p><p>91   The <em class="parameter"><code>indexPages</code></em> should be set to the number of leaf pages.92   This is used to estimate the number of workers for parallel index scan.93  </p><p>94   When <em class="parameter"><code>loop_count</code></em> is greater than one, the returned numbers95   should be averages expected for any one scan of the index.96  </p><div class="procedure" id="id-1.10.15.12.13"><p class="title"><strong>Cost Estimation</strong></p><p>97    A typical cost estimator will proceed as follows:98   </p><ol class="procedure" type="1"><li class="step"><p>99     Estimate and return the fraction of parent-table rows that will be visited100     based on the given qual conditions.  In the absence of any index-type-specific101     knowledge, use the standard optimizer function <code class="function">clauselist_selectivity()</code>:102 103</p><pre class="programlisting">104*indexSelectivity = clauselist_selectivity(root, path-&gt;indexquals,105                                           path-&gt;indexinfo-&gt;rel-&gt;relid,106                                           JOIN_INNER, NULL);107</pre><p>108    </p></li><li class="step"><p>109     Estimate the number of index rows that will be visited during the110     scan.  For many index types this is the same as <em class="parameter"><code>indexSelectivity</code></em> times111     the number of rows in the index, but it might be more.  (Note that the112     index's size in pages and rows is available from the113     <code class="literal">path-&gt;indexinfo</code> struct.)114    </p></li><li class="step"><p>115     Estimate the number of index pages that will be retrieved during the scan.116     This might be just <em class="parameter"><code>indexSelectivity</code></em> times the index's size in pages.117    </p></li><li class="step"><p>118     Compute the index access cost.  A generic estimator might do this:119 120</p><pre class="programlisting">121/*122 * Our generic assumption is that the index pages will be read123 * sequentially, so they cost seq_page_cost each, not random_page_cost.124 * Also, we charge for evaluation of the indexquals at each index row.125 * All the costs are assumed to be paid incrementally during the scan.126 */127cost_qual_eval(&amp;index_qual_cost, path-&gt;indexquals, root);128*indexStartupCost = index_qual_cost.startup;129*indexTotalCost = seq_page_cost * numIndexPages +130    (cpu_index_tuple_cost + index_qual_cost.per_tuple) * numIndexTuples;131</pre><p>132 133     However, the above does not account for amortization of index reads134     across repeated index scans.135    </p></li><li class="step"><p>136     Estimate the index correlation.  For a simple ordered index on a single137     field, this can be retrieved from pg_statistic.  If the correlation138     is not known, the conservative estimate is zero (no correlation).139    </p></li></ol></div><p>140   Examples of cost estimator functions can be found in141   <code class="filename">src/backend/utils/adt/selfuncs.c</code>.142  </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="index-unique-checks.html" title="64.5. Index Uniqueness Checks">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="indexam.html" title="Chapter 64. Index Access Method Interface Definition">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="generic-wal.html" title="Chapter 65. Generic WAL Records">Next</a></td></tr><tr><td width="40%" align="left" valign="top">64.5. Index Uniqueness Checks </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="40%" align="right" valign="top"> Chapter 65. Generic WAL Records</td></tr></table></div></body></html>