there are other methods available. The
        default is <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-sort" xreflabel="enable_sort">
      <term><varname>enable_sort</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>enable_sort</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables or disables the query planner's use of explicit sort
        steps. It is impossible to suppress explicit sorts entirely,
        but turning this variable off discourages the planner from
        using one if there are other methods available. The default
        is <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-tidscan" xreflabel="enable_tidscan">
      <term><varname>enable_tidscan</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>enable_tidscan</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables or disables the query planner's use of <acronym>TID</acronym>
        scan plan types. The default is <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
     </sect2>
     <sect2 id="runtime-config-query-constants">
     <title>Planner Cost Constants</title>

    <para>
     The <firstterm>cost</firstterm> variables described in this section are measured
     on an arbitrary scale.  Only their relative values matter, hence
     scaling them all up or down by the same factor will result in no change
     in the planner's choices.  By default, these cost variables are based on
     the cost of sequential page fetches; that is,
     <varname>seq_page_cost</varname> is conventionally set to <literal>1.0</literal>
     and the other cost variables are set with reference to that.  But
     you can use a different scale if you prefer, such as actual execution
     times in milliseconds on a particular machine.
    </para>

   <note>
    <para>
     Unfortunately, there is no well-defined method for determining ideal
     values for the cost variables.  They are best treated as averages over
     the entire mix of queries that a particular installation will receive.  This
     means that changing them on the basis of just a few experiments is very
     risky.
    </para>
   </note>

     <variablelist>

     <varlistentry id="guc-seq-page-cost" xreflabel="seq_page_cost">
      <term><varname>seq_page_cost</varname> (<type>floating point</type>)
      <indexterm>
       <primary><varname>seq_page_cost</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the planner's estimate of the cost of a disk page fetch
        that is part of a series of sequential fetches.  The default is 1.0.
        This value can be overridden for tables and indexes in a particular
        tablespace by setting the tablespace parameter of the same name
        (see <xref linkend="sql-altertablespace"/>).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-random-page-cost" xreflabel="random_page_cost">
      <term><varname>random_page_cost</varname> (<type>floating point</type>)
      <indexterm>
       <primary><varname>random_page_cost</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the planner's estimate of the cost of a
        non-sequentially-fetched disk page.  The default is 4.0.
        This value can be overridden for tables and indexes in a particular
        tablespace by setting the tablespace parameter of the same name
        (see <xref linkend="sql-altertablespace"/>).
       </para>

       <para>
        Reducing this value relative to <varname>seq_page_cost</varname>
        will cause the system to prefer index scans; raising it will