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<!--
$PostgreSQL: pgsql/doc/src/sgml/ref/prepare_transaction.sgml,v 1.3 2005/08/30 01:37:38 tgl Exp $
PostgreSQL documentation
-->
<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 <command>COMMIT PREPARED</command> or
<command>ROLLBACK PREPARED</command>, 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
<command>BEGIN</command> to start one.
</para>
<para>
It is not currently allowed to <command>PREPARE</> a transaction that
has executed any operations involving temporary tables or
created any cursors <literal>WITH HOLD</>. 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</>,
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
<structname>pg_prepared_xacts</> 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>
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