<!--
doc/src/sgml/ref/alter_system.sgml
PostgreSQL documentation
-->

<refentry id="SQL-ALTERSYSTEM">
 <indexterm zone="sql-altersystem">
  <primary>ALTER SYSTEM</primary>
 </indexterm>

 <refmeta>
  <refentrytitle>ALTER SYSTEM</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>ALTER SYSTEM</refname>
  <refpurpose>change a server configuration parameter</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
ALTER SYSTEM SET <replaceable class="PARAMETER">configuration_parameter</replaceable> { TO | = } { <replaceable class="PARAMETER">value</replaceable> | '<replaceable class="PARAMETER">value</replaceable>' | DEFAULT }

ALTER SYSTEM RESET <replaceable class="PARAMETER">configuration_parameter</replaceable>
ALTER SYSTEM RESET ALL
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>ALTER SYSTEM</command> is used for changing server configuration
   parameters across the entire database cluster.  It can be more convenient
   than the traditional method of manually editing
   the <filename>postgresql.conf</filename> file.
   <command>ALTER SYSTEM</command> writes the given parameter setting to
   the <filename>postgresql.auto.conf</filename> file, which is read in
   addition to <filename>postgresql.conf</filename>.
   Setting a parameter to <literal>DEFAULT</literal>, or using the
   <command>RESET</command> variant, removes that configuration entry from the
   <filename>postgresql.auto.conf</filename> file. Use <literal>RESET
   ALL</literal> to remove all such configuration entries.
  </para>

  <para>
   Values set with <command>ALTER SYSTEM</command> will be effective after
   the next server configuration reload (<systemitem>SIGHUP</>
   or <literal>pg_ctl reload</>), or after the next server restart in the
   case of parameters that can only be changed at server start.
  </para>

  <para>
   Only superusers can use <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.
      <literal>DEFAULT</literal> can be written to specify removing the
      parameter and its value from <filename>postgresql.auto.conf</filename>.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Notes</title>

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

  <para>
   See <xref linkend="config-setting"> for other ways to set the parameters.
  </para>
 </refsect1>

 <refsect1>
  <title>Examples</title>

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

  <para>
   Undo that, restoring whatever setting was effective
   in <filename>postgresql.conf</>:
<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>
