identifiers or numbers must be single-quoted.  To embed a single
     quote in a parameter value, write either two quotes (preferred)
     or backslash-quote.
     If the file contains multiple entries for the same parameter,
     all but the last one are ignored.
    </para>

    <para>
     Parameters set in this way provide default values for the cluster.
     The settings seen by active sessions will be these values unless they
     are overridden.  The following sections describe ways in which the
     administrator or user can override these defaults.
    </para>

    <para>
     <indexterm>
      <primary>SIGHUP</primary>
     </indexterm>
     The configuration file is reread whenever the main server process
     receives a <systemitem>SIGHUP</systemitem> signal; this signal is most easily
     sent by running <literal>pg_ctl reload</literal> from the command line or by
     calling the SQL function <function>pg_reload_conf()</function>. The main
     server process also propagates this signal to all currently running
     server processes, so that existing sessions also adopt the new values
     (this will happen after they complete any currently-executing client
     command).  Alternatively, you can
     send the signal to a single server process directly.  Some parameters
     can only be set at server start; any changes to their entries in the
     configuration file will be ignored until the server is restarted.
     Invalid parameter settings in the configuration file are likewise
     ignored (but logged) during <systemitem>SIGHUP</systemitem> processing.
    </para>

    <para>
     In addition to <filename>postgresql.conf</filename>,
     a <productname>PostgreSQL</productname> data directory contains a file
     <filename>postgresql.auto.conf</filename><indexterm><primary>postgresql.auto.conf</primary></indexterm>,
     which has the same format as <filename>postgresql.conf</filename> but
     is intended to be edited automatically, not manually.  This file holds
     settings provided through the <link linkend="sql-altersystem"><command>ALTER SYSTEM</command></link> command.
     This file is read whenever <filename>postgresql.conf</filename> is,
     and its settings take effect in the same way.  Settings
     in <filename>postgresql.auto.conf</filename> override those
     in <filename>postgresql.conf</filename>.
    </para>

    <para>
     External tools may also
     modify <filename>postgresql.auto.conf</filename>.  It is not
     recommended to do this while the server is running unless <xref
     linkend="guc-allow-alter-system"/> is set to <literal>off</literal>, since a
     concurrent <command>ALTER SYSTEM</command> command could overwrite
     such changes.  Such tools might simply append new settings to the end,
     or they might choose to remove duplicate settings and/or comments
     (as <command>ALTER SYSTEM</command> will).
    </para>

    <para>
     The system view
     <link linkend="view-pg-file-settings"><structname>pg_file_settings</structname></link>
     can be helpful for pre-testing changes to the configuration files, or for
     diagnosing problems if a <systemitem>SIGHUP</systemitem> signal did not have the
     desired effects.
    </para>
   </sect2>

   <sect2 id="config-setting-sql">
    <title>Parameter Interaction via SQL</title>

     <para>
      <productname>PostgreSQL</productname> provides three SQL
      commands to establish configuration defaults.
      The already-mentioned <command>ALTER SYSTEM</command> command
      provides an SQL-accessible means of changing global defaults; it is
      functionally equivalent to editing <filename>postgresql.conf</filename>.
      In addition, there are two commands that allow setting of defaults
      on a per-database or per-role basis:
     </para>

     <itemizedlist>
     <listitem>
      <para>
       The <link linkend="sql-alterdatabase"><command>ALTER DATABASE</command></link> command allows global
       settings to