id="runtime-config-query-geqo">
     <title>Genetic Query Optimizer</title>

     <para>
      The genetic query optimizer (GEQO) is an algorithm that does query
      planning using heuristic searching.  This reduces planning time for
      complex queries (those joining many relations), at the cost of producing
      plans that are sometimes inferior to those found by the normal
      exhaustive-search algorithm.
      For more information see <xref linkend="geqo"/>.
     </para>

     <variablelist>

     <varlistentry id="guc-geqo" xreflabel="geqo">
      <term><varname>geqo</varname> (<type>boolean</type>)
      <indexterm>
       <primary>genetic query optimization</primary>
      </indexterm>
      <indexterm>
       <primary>GEQO</primary>
       <see>genetic query optimization</see>
      </indexterm>
      <indexterm>
       <primary><varname>geqo</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables or disables genetic query optimization.
        This is on by default.  It is usually best not to turn it off in
        production; the <varname>geqo_threshold</varname> variable provides
        more granular control of GEQO.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-geqo-threshold" xreflabel="geqo_threshold">
      <term><varname>geqo_threshold</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>geqo_threshold</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Use genetic query optimization to plan queries with at least
        this many <literal>FROM</literal> items involved. (Note that a
        <literal>FULL OUTER JOIN</literal> construct counts as only one <literal>FROM</literal>
        item.) The default is 12. For simpler queries it is usually best
        to use the regular, exhaustive-search planner, but for queries with
        many tables the exhaustive search takes too long, often
        longer than the penalty of executing a suboptimal plan.  Thus,
        a threshold on the size of the query is a convenient way to manage
        use of GEQO.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-geqo-effort" xreflabel="geqo_effort">
      <term><varname>geqo_effort</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>geqo_effort</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Controls the trade-off between planning time and query plan
        quality in GEQO. This variable must be an integer in the
        range from 1 to 10. The default value is five. Larger values
        increase the time spent doing query planning, but also
        increase the likelihood that an efficient query plan will be
        chosen.
       </para>

       <para>
        <varname>geqo_effort</varname> doesn't actually do anything
        directly; it is only used to compute the default values for
        the other variables that influence GEQO behavior (described
        below). If you prefer, you can set the other parameters by
        hand instead.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-geqo-pool-size" xreflabel="geqo_pool_size">
      <term><varname>geqo_pool_size</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>geqo_pool_size</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Controls the pool size used by GEQO, that is the
        number of individuals in the genetic population.  It must be
        at least two, and useful values are typically 100 to 1000.  If
        it is set to zero (the default setting) then a suitable
        value is chosen based on <varname>geqo_effort</varname> and
        the number of tables in the query.
       </para>
      </listitem>
     </varlistentry>