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<!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>62.3. 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 Vsnapshot" /><link rel="prev" href="geqo-intro2.html" title="62.2. Genetic Algorithms" /><link rel="next" href="geqo-biblio.html" title="62.4. Further Reading" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">62.3. Genetic Query Optimization (<acronym class="acronym">GEQO</acronym>) in PostgreSQL</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="geqo-intro2.html" title="62.2. Genetic Algorithms">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="geqo.html" title="Chapter 62. Genetic Query Optimizer">Up</a></td><th width="60%" align="center">Chapter 62. Genetic Query Optimizer</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="geqo-biblio.html" title="62.4. Further Reading">Next</a></td></tr></table><hr /></div><div class="sect1" id="GEQO-PG-INTRO"><div class="titlepage"><div><div><h2 class="title" style="clear: both">62.3. 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-1.10.13.5.6">62.3.1. Generating Possible Plans with <acronym class="acronym">GEQO</acronym></a></span></dt><dt><span class="sect2"><a href="geqo-pg-intro.html#GEQO-FUTURE">62.3.2. 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-1.10.13.5.6"><div class="titlepage"><div><div><h3 class="title">62.3.1. 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">62.3.2. 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 class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="geqo-intro2.html" title="62.2. Genetic Algorithms">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="geqo.html" title="Chapter 62. Genetic Query Optimizer">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="geqo-biblio.html" title="62.4. Further Reading">Next</a></td></tr><tr><td width="40%" align="left" valign="top">62.2. Genetic Algorithms </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 62.4. Further Reading</td></tr></table></div></body></html>