involving temporary tables or the session's
   temporary namespace, created any cursors <literal>WITH HOLD</literal>, or
   executed <command>LISTEN</command>, <command>UNLISTEN</command>, or
   <command>NOTIFY</command>.
   Those features are too tightly
   tied to the current session to be useful in a transaction to be prepared.
  </para>

  <para>
   If the transaction modified any run-time parameters with <command>SET</command>
   (without the <literal>LOCAL</literal> option),
   those effects persist after <command>PREPARE TRANSACTION</command>, and will not
   be affected by any later <command>COMMIT PREPARED</command> or
   <command>ROLLBACK PREPARED</command>.  Thus, in this one respect
   <command>PREPARE TRANSACTION</command> acts more like <command>COMMIT</command> than
   <command>ROLLBACK</command>.
  </para>

  <para>
   All currently available prepared transactions are listed in the
   <link linkend="view-pg-prepared-xacts"><structname>pg_prepared_xacts</structname></link>
   system view.
  </para>

  <caution>
   <para>
    It is unwise to leave transactions in the prepared state for a long time.
    This will interfere with the ability of <command>VACUUM</command> to reclaim
    storage, and in extreme cases could cause the database to shut down
    to prevent transaction ID wraparound (see <xref
    linkend="vacuum-for-wraparound"/>).  Keep in mind also that the transaction
    continues to hold whatever locks it held.  The intended usage of the
    feature is that a prepared transaction will normally be committed or
    rolled back as soon as an external transaction manager has verified that
    other databases are also prepared to commit.
   </para>

   <para>
    If you have not set up an external transaction manager to track prepared
    transactions and ensure they get closed out promptly, it is best to keep
    the prepared-transaction feature disabled by setting
    <xref linkend="guc-max-prepared-transactions"/> to zero.  This will
    prevent accidental creation of prepared transactions that might then
    be forgotten and eventually cause problems.
   </para>
  </caution>
 </refsect1>

 <refsect1 id="sql-prepare-transaction-examples">
  <title>Examples</title>
  <para>
   Prepare the current transaction for two-phase commit, using
   <literal>foobar</literal> as the transaction identifier:

<programlisting>
PREPARE TRANSACTION 'foobar';
</programlisting></para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   <command>PREPARE TRANSACTION</command> is a
   <productname>PostgreSQL</productname> extension.  It is intended for use by
   external transaction management systems, some of which are covered by
   standards (such as X/Open XA), but the SQL side of those systems is not
   standardized.
  </para>
 </refsect1>

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

  <simplelist type="inline">
   <member><xref linkend="sql-commit-prepared"/></member>
   <member><xref linkend="sql-rollback-prepared"/></member>
  </simplelist>
 </refsect1>

</refentry>