libraries might provide their own mechanisms,
      via the shell or otherwise, that allow the user to alter session
      settings without direct use of SQL commands.
     </para>
    </listitem>
   </itemizedlist>

   </sect2>

   <sect2 id="config-includes">
    <title>Managing Configuration File Contents</title>

     <para>
      <productname>PostgreSQL</productname> provides several features for breaking
      down complex <filename>postgresql.conf</filename> files into sub-files.
      These features are especially useful when managing multiple servers
      with related, but not identical, configurations.
     </para>

     <para>
      <indexterm>
       <primary><literal>include</literal></primary>
       <secondary>in configuration file</secondary>
      </indexterm>
      In addition to individual parameter settings,
      the <filename>postgresql.conf</filename> file can contain <firstterm>include
      directives</firstterm>, which specify another file to read and process as if
      it were inserted into the configuration file at this point.  This
      feature allows a configuration file to be divided into physically
      separate parts.  Include directives simply look like:
<programlisting>
include 'filename'
</programlisting>
      If the file name is not an absolute path, it is taken as relative to
      the directory containing the referencing configuration file.
      Inclusions can be nested.
     </para>

     <para>
      <indexterm>
       <primary><literal>include_if_exists</literal></primary>
       <secondary>in configuration file</secondary>
      </indexterm>
      There is also an <literal>include_if_exists</literal> directive, which acts
      the same as the <literal>include</literal> directive, except
      when the referenced file does not exist or cannot be read.  A regular
      <literal>include</literal> will consider this an error condition, but
      <literal>include_if_exists</literal> merely logs a message and continues
      processing the referencing configuration file.
     </para>

     <para>
      <indexterm>
       <primary><literal>include_dir</literal></primary>
       <secondary>in configuration file</secondary>
      </indexterm>
      The <filename>postgresql.conf</filename> file can also contain
      <literal>include_dir</literal> directives, which specify an entire
      directory of configuration files to include.  These look like
<programlisting>
include_dir 'directory'
</programlisting>
      Non-absolute directory names are taken as relative to the directory
      containing the referencing configuration file.  Within the specified
      directory, only non-directory files whose names end with the
      suffix <literal>.conf</literal> will be included.  File names that
      start with the <literal>.</literal> character are also ignored, to
      prevent mistakes since such files are hidden on some platforms.  Multiple
      files within an include directory are processed in file name order
      (according to C locale rules, i.e., numbers before letters, and
      uppercase letters before lowercase ones).
     </para>

     <para>
      Include files or directories can be used to logically separate portions
      of the database configuration, rather than having a single large
      <filename>postgresql.conf</filename> file.  Consider a company that has two
      database servers, each with a different amount of memory.  There are
      likely elements of the configuration both will share, for things such
      as logging.  But memory-related parameters on the server will vary
      between the two.  And there might be server specific customizations,
      too.  One way to manage this situation is to break the custom
      configuration changes for your site into three files.  You could add
      this to the end of your <filename>postgresql.conf</filename> file to include
      them:
<programlisting>
include 'shared.conf'
include 'memory.conf'
include 'server.conf'