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$PostgreSQL: pgsql/doc/src/sgml/ref/prepare_transaction.sgml,v 1.7 2008/03/12 20:11:45 tgl Exp $
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<refentry id="SQL-PREPARE-TRANSACTION">
 <refmeta>
  <refentrytitle id="sql-prepare-transaction-title">PREPARE TRANSACTION</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

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

 <indexterm zone="sql-prepare-transaction">
  <primary>PREPARE TRANSACTION</primary>
 </indexterm>

 <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" endterm="sql-commit-prepared-title">
   or <xref linkend="sql-rollback-prepared" endterm="sql-rollback-prepared-title">,
   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>
   This command must be used inside a transaction block. Use <xref
   linkend="sql-begin" endterm="sql-begin-title"> 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>

  <para>
   From a performance standpoint, it is unwise to leave transactions in
   the prepared state for a long time: this will for instance interfere with
   the ability of <command>VACUUM</> to reclaim storage.  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 make any serious use of prepared transactions, you will probably
   want to increase the value of <xref
   linkend="guc-max-prepared-transactions">, as the default setting is
   quite small (to avoid wasting resources for those who don't use it).
   It is recommendable to make it at least equal to
   <xref linkend="guc-max-connections">, so that every session can have
   a prepared transaction pending.
  </para>
 </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>See Also</title>

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

</refentry>
