entries.
  </para>

  <para>
   Values set with <command>ALTER SYSTEM</command> will be effective after
   the next server configuration reload, or after the next server restart
   in the case of parameters that can only be changed at server start.
   A server configuration reload can be commanded by calling the SQL
   function <function>pg_reload_conf()</function>, running <literal>pg_ctl reload</literal>,
   or sending a <systemitem>SIGHUP</systemitem> signal to the main server process.
  </para>

  <para>
   Only superusers and users granted <literal>ALTER SYSTEM</literal> privilege
   on a parameter can change it using <command>ALTER SYSTEM</command>.  Also, since
   this command acts directly on the file system and cannot be rolled back,
   it is not allowed inside a transaction block or function.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><replaceable class="parameter">configuration_parameter</replaceable></term>
    <listitem>
     <para>
      Name of a settable configuration parameter.  Available parameters are
      documented in <xref linkend="runtime-config"/>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">value</replaceable></term>
    <listitem>
     <para>
      New value of the parameter.  Values can be specified as string
      constants, identifiers, numbers, or comma-separated lists of
      these, as appropriate for the particular parameter.
      Values that are neither numbers nor valid identifiers must be quoted.
      <literal>DEFAULT</literal> can be written to specify removing the
      parameter and its value from <filename>postgresql.auto.conf</filename>.
     </para>

     <para>
      For some list-accepting parameters, quoted values will produce
      double-quoted output to preserve whitespace and commas; for others,
      double-quotes must be used inside single-quoted strings to get
      this effect.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   This command can't be used to set <xref linkend="guc-data-directory"/>,
   <xref linkend="guc-allow-alter-system"/>,
   nor parameters that are not allowed in <filename>postgresql.conf</filename>
   (e.g., <link linkend="runtime-config-preset">preset options</link>).
  </para>

  <para>
   See <xref linkend="config-setting"/> for other ways to set the parameters.
  </para>

  <para>
   <literal>ALTER SYSTEM</literal> can be disabled by setting
   <xref linkend="guc-allow-alter-system"/> to <literal>off</literal>, but this
   is not a security mechanism (as explained in detail in the documentation for
   this parameter).
  </para>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   Set the <literal>wal_level</literal>:
<programlisting>
ALTER SYSTEM SET wal_level = replica;
</programlisting>
  </para>

  <para>
   Undo that, restoring whatever setting was effective
   in <filename>postgresql.conf</filename>:
<programlisting>
ALTER SYSTEM RESET wal_level;
</programlisting></para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   The <command>ALTER SYSTEM</command> statement is a
   <productname>PostgreSQL</productname> extension.
  </para>
 </refsect1>

 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-set"/></member>
   <member><xref linkend="sql-show"/></member>
  </simplelist>
 </refsect1>

</refentry>