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diff --git a/doc/src/sgml/html/geqo-pg-intro.html b/doc/src/sgml/html/geqo-pg-intro.html new file mode 100644 index 0000000..f10da6b --- /dev/null +++ b/doc/src/sgml/html/geqo-pg-intro.html @@ -0,0 +1,107 @@ +<?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>59.3. Genetic Query Optimization (GEQO) in PostgreSQL</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 V1.79.1" /><link rel="prev" href="geqo-intro2.html" title="59.2. Genetic Algorithms" /><link rel="next" href="geqo-biblio.html" title="59.4. Further Reading" /></head><body id="docContent" class="container-fluid col-10"><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">59.3. Genetic Query Optimization (<acronym xmlns="http://www.w3.org/1999/xhtml" class="acronym">GEQO</acronym>) in PostgreSQL</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="geqo-intro2.html" title="59.2. Genetic Algorithms">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="geqo.html" title="Chapter 59. Genetic Query Optimizer">Up</a></td><th width="60%" align="center">Chapter 59. Genetic Query Optimizer</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="geqo-biblio.html" title="59.4. Further Reading">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="GEQO-PG-INTRO"><div class="titlepage"><div><div><h2 class="title" style="clear: both">59.3. Genetic Query Optimization (<acronym class="acronym">GEQO</acronym>) in PostgreSQL</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="geqo-pg-intro.html#id-1.10.12.5.6">59.3.1. Generating Possible Plans with <acronym class="acronym">GEQO</acronym></a></span></dt><dt><span class="sect2"><a href="geqo-pg-intro.html#GEQO-FUTURE">59.3.2. Future Implementation Tasks for + <span class="productname">PostgreSQL</span> <acronym class="acronym">GEQO</acronym></a></span></dt></dl></div><p> + The <acronym class="acronym">GEQO</acronym> module approaches the query + optimization problem as though it were the well-known traveling salesman + problem (<acronym class="acronym">TSP</acronym>). + Possible query plans are encoded as integer strings. Each string + represents the join order from one relation of the query to the next. + For example, the join tree +</p><pre class="literallayout"> + /\ + /\ 2 + /\ 3 +4 1 +</pre><p> + is encoded by the integer string '4-1-3-2', + which means, first join relation '4' and '1', then '3', and + then '2', where 1, 2, 3, 4 are relation IDs within the + <span class="productname">PostgreSQL</span> optimizer. + </p><p> + Specific characteristics of the <acronym class="acronym">GEQO</acronym> + implementation in <span class="productname">PostgreSQL</span> + are: + + </p><div class="itemizedlist"><ul class="itemizedlist compact" style="list-style-type: bullet; "><li class="listitem" style="list-style-type: disc"><p> + Usage of a <em class="firstterm">steady state</em> <acronym class="acronym">GA</acronym> (replacement of the least fit + individuals in a population, not whole-generational replacement) + allows fast convergence towards improved query plans. This is + essential for query handling with reasonable time; + </p></li><li class="listitem" style="list-style-type: disc"><p> + Usage of <em class="firstterm">edge recombination crossover</em> + which is especially suited to keep edge losses low for the + solution of the <acronym class="acronym">TSP</acronym> by means of a + <acronym class="acronym">GA</acronym>; + </p></li><li class="listitem" style="list-style-type: disc"><p> + Mutation as genetic operator is deprecated so that no repair + mechanisms are needed to generate legal <acronym class="acronym">TSP</acronym> tours. + </p></li></ul></div><p> + </p><p> + Parts of the <acronym class="acronym">GEQO</acronym> module are adapted from D. Whitley's + Genitor algorithm. + </p><p> + The <acronym class="acronym">GEQO</acronym> module allows + the <span class="productname">PostgreSQL</span> query optimizer to + support large join queries effectively through + non-exhaustive search. + </p><div class="sect2" id="id-1.10.12.5.6"><div class="titlepage"><div><div><h3 class="title">59.3.1. Generating Possible Plans with <acronym class="acronym">GEQO</acronym></h3></div></div></div><p> + The <acronym class="acronym">GEQO</acronym> planning process uses the standard planner + code to generate plans for scans of individual relations. Then join + plans are developed using the genetic approach. As shown above, each + candidate join plan is represented by a sequence in which to join + the base relations. In the initial stage, the <acronym class="acronym">GEQO</acronym> + code simply generates some possible join sequences at random. For each + join sequence considered, the standard planner code is invoked to + estimate the cost of performing the query using that join sequence. + (For each step of the join sequence, all three possible join strategies + are considered; and all the initially-determined relation scan plans + are available. The estimated cost is the cheapest of these + possibilities.) Join sequences with lower estimated cost are considered + <span class="quote">“<span class="quote">more fit</span>”</span> than those with higher cost. The genetic algorithm + discards the least fit candidates. Then new candidates are generated + by combining genes of more-fit candidates — that is, by using + randomly-chosen portions of known low-cost join sequences to create + new sequences for consideration. This process is repeated until a + preset number of join sequences have been considered; then the best + one found at any time during the search is used to generate the finished + plan. + </p><p> + This process is inherently nondeterministic, because of the randomized + choices made during both the initial population selection and subsequent + <span class="quote">“<span class="quote">mutation</span>”</span> of the best candidates. To avoid surprising changes + of the selected plan, each run of the GEQO algorithm restarts its + random number generator with the current <a class="xref" href="runtime-config-query.html#GUC-GEQO-SEED">geqo_seed</a> + parameter setting. As long as <code class="varname">geqo_seed</code> and the other + GEQO parameters are kept fixed, the same plan will be generated for a + given query (and other planner inputs such as statistics). To experiment + with different search paths, try changing <code class="varname">geqo_seed</code>. + </p></div><div class="sect2" id="GEQO-FUTURE"><div class="titlepage"><div><div><h3 class="title">59.3.2. Future Implementation Tasks for + <span class="productname">PostgreSQL</span> <acronym class="acronym">GEQO</acronym></h3></div></div></div><p> + Work is still needed to improve the genetic algorithm parameter + settings. + In file <code class="filename">src/backend/optimizer/geqo/geqo_main.c</code>, + routines + <code class="function">gimme_pool_size</code> and <code class="function">gimme_number_generations</code>, + we have to find a compromise for the parameter settings + to satisfy two competing demands: + </p><div class="itemizedlist"><ul class="itemizedlist compact" style="list-style-type: disc; "><li class="listitem"><p> + Optimality of the query plan + </p></li><li class="listitem"><p> + Computing time + </p></li></ul></div><p> + </p><p> + In the current implementation, the fitness of each candidate join + sequence is estimated by running the standard planner's join selection + and cost estimation code from scratch. To the extent that different + candidates use similar sub-sequences of joins, a great deal of work + will be repeated. This could be made significantly faster by retaining + cost estimates for sub-joins. The problem is to avoid expending + unreasonable amounts of memory on retaining that state. + </p><p> + At a more basic level, it is not clear that solving query optimization + with a GA algorithm designed for TSP is appropriate. In the TSP case, + the cost associated with any substring (partial tour) is independent + of the rest of the tour, but this is certainly not true for query + optimization. Thus it is questionable whether edge recombination + crossover is the most effective mutation procedure. + </p></div></div><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="geqo-intro2.html" title="59.2. 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