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doc/src/sgml/ref/prepare_transaction.sgml
PostgreSQL documentation
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<refentry id="SQL-PREPARE-TRANSACTION">
 <indexterm zone="sql-prepare-transaction">
  <primary>PREPARE TRANSACTION</primary>
 </indexterm>

 <refmeta>
  <refentrytitle>PREPARE TRANSACTION</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>PREPARE TRANSACTION</refname>
  <refpurpose>prepare the current transaction for two-phase commit</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
PREPARE TRANSACTION <replaceable class="PARAMETER">transaction_id</replaceable>
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>PREPARE TRANSACTION</command> prepares the current transaction
   for two-phase commit. After this command, the transaction is no longer
   associated with the current session; instead, its state is fully stored on
   disk, and there is a very high probability that it can be committed
   successfully, even if a database crash occurs before the commit is
   requested.
  </para>

  <para>
   Once prepared, a transaction can later be committed or rolled back
   with <xref linkend="sql-commit-prepared">
   or <xref linkend="sql-rollback-prepared">,
   respectively.  Those commands can be issued from any session, not
   only the one that executed the original transaction.
  </para>

  <para>
   From the point of view of the issuing session, <command>PREPARE
   TRANSACTION</command> is not unlike a <command>ROLLBACK</> command:
   after executing it, there is no active current transaction, and the
   effects of the prepared transaction are no longer visible.  (The effects
   will become visible again if the transaction is committed.)
  </para>

  <para>
   If the <command>PREPARE TRANSACTION</command> command fails for any
   reason, it becomes a <command>ROLLBACK</>: the current transaction
   is canceled.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><replaceable class="PARAMETER">transaction_id</replaceable></term>
    <listitem>
     <para>
      An arbitrary identifier that later identifies this transaction for
      <command>COMMIT PREPARED</> or <command>ROLLBACK PREPARED</>.
      The identifier must be written as a string literal, and must be
      less than 200 bytes long.  It must not be the same as the identifier
      used for any currently prepared transaction.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   <command>PREPARE TRANSACTION</> is not intended for use in applications
   or interactive sessions. Its purpose is to allow an external
   transaction manager to perform atomic global transactions across multiple
   databases or other transactional resources. Unless you're writing a
   transaction manager, you probably shouldn't be using <command>PREPARE
   TRANSACTION</>.
  </para>

  <para>
   This command must be used inside a transaction block. Use <xref
   linkend="sql-begin"> to start one.
  </para>

  <para>
   It is not currently allowed to <command>PREPARE</> a transaction that
   has executed any operations involving temporary tables,
   created any cursors <literal>WITH HOLD</>, or executed
   <command>LISTEN</> or <command>UNLISTEN</>.
   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</>
   (without the <literal>LOCAL</> option),
   those effects persist after <command>PREPARE TRANSACTION</>, 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</> acts more like <command>COMMIT</> than
   <command>ROLLBACK</>.
  </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</> 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 id="sql-prepare-transaction-examples-title">Examples</title>
  <para>
   Prepare the current transaction for two-phase commit, using
   <literal>foobar</> 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>
