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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>PREPARE TRANSACTION</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="sql-prepare.html" title="PREPARE" /><link rel="next" href="sql-reassign-owned.html" title="REASSIGN OWNED" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">PREPARE TRANSACTION</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="sql-prepare.html" title="PREPARE">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><th width="60%" align="center">SQL Commands</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="sql-reassign-owned.html" title="REASSIGN OWNED">Next</a></td></tr></table><hr></hr></div><div class="refentry" id="SQL-PREPARE-TRANSACTION"><div class="titlepage"></div><a id="id-1.9.3.159.1" class="indexterm"></a><div class="refnamediv"><h2><span class="refentrytitle">PREPARE TRANSACTION</span></h2><p>PREPARE TRANSACTION — prepare the current transaction for two-phase commit</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><pre class="synopsis">
PREPARE TRANSACTION <em class="replaceable"><code>transaction_id</code></em>
</pre></div><div class="refsect1" id="id-1.9.3.159.5"><h2>Description</h2><p>
<code class="command">PREPARE TRANSACTION</code> 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.
</p><p>
Once prepared, a transaction can later be committed or rolled back
with <a class="xref" href="sql-commit-prepared.html" title="COMMIT PREPARED"><span class="refentrytitle">COMMIT PREPARED</span></a>
or <a class="xref" href="sql-rollback-prepared.html" title="ROLLBACK PREPARED"><span class="refentrytitle">ROLLBACK PREPARED</span></a>,
respectively. Those commands can be issued from any session, not
only the one that executed the original transaction.
</p><p>
From the point of view of the issuing session, <code class="command">PREPARE
TRANSACTION</code> is not unlike a <code class="command">ROLLBACK</code> 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.)
</p><p>
If the <code class="command">PREPARE TRANSACTION</code> command fails for any
reason, it becomes a <code class="command">ROLLBACK</code>: the current transaction
is canceled.
</p></div><div class="refsect1" id="id-1.9.3.159.6"><h2>Parameters</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="replaceable"><code>transaction_id</code></em></span></dt><dd><p>
An arbitrary identifier that later identifies this transaction for
<code class="command">COMMIT PREPARED</code> or <code class="command">ROLLBACK PREPARED</code>.
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.
</p></dd></dl></div></div><div class="refsect1" id="id-1.9.3.159.7"><h2>Notes</h2><p>
<code class="command">PREPARE TRANSACTION</code> 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 <code class="command">PREPARE
TRANSACTION</code>.
</p><p>
This command must be used inside a transaction block. Use <a class="xref" href="sql-begin.html" title="BEGIN"><span class="refentrytitle">BEGIN</span></a> to start one.
</p><p>
It is not currently allowed to <code class="command">PREPARE</code> a transaction that
has executed any operations involving temporary tables or the session's
temporary namespace, created any cursors <code class="literal">WITH HOLD</code>, or
executed <code class="command">LISTEN</code>, <code class="command">UNLISTEN</code>, or
<code class="command">NOTIFY</code>.
Those features are too tightly
tied to the current session to be useful in a transaction to be prepared.
</p><p>
If the transaction modified any run-time parameters with <code class="command">SET</code>
(without the <code class="literal">LOCAL</code> option),
those effects persist after <code class="command">PREPARE TRANSACTION</code>, and will not
be affected by any later <code class="command">COMMIT PREPARED</code> or
<code class="command">ROLLBACK PREPARED</code>. Thus, in this one respect
<code class="command">PREPARE TRANSACTION</code> acts more like <code class="command">COMMIT</code> than
<code class="command">ROLLBACK</code>.
</p><p>
All currently available prepared transactions are listed in the
<a class="link" href="view-pg-prepared-xacts.html" title="52.77. pg_prepared_xacts"><code class="structname">pg_prepared_xacts</code></a>
system view.
</p><div class="caution"><h3 class="title">Caution</h3><p>
It is unwise to leave transactions in the prepared state for a long time.
This will interfere with the ability of <code class="command">VACUUM</code> to reclaim
storage, and in extreme cases could cause the database to shut down
to prevent transaction ID wraparound (see <a class="xref" href="routine-vacuuming.html#VACUUM-FOR-WRAPAROUND" title="24.1.5. Preventing Transaction ID Wraparound Failures">Section 24.1.5</a>). 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.
</p><p>
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
<a class="xref" href="runtime-config-resource.html#GUC-MAX-PREPARED-TRANSACTIONS">max_prepared_transactions</a> to zero. This will
prevent accidental creation of prepared transactions that might then
be forgotten and eventually cause problems.
</p></div></div><div class="refsect1" id="SQL-PREPARE-TRANSACTION-EXAMPLES"><h2>Examples</h2><p>
Prepare the current transaction for two-phase commit, using
<code class="literal">foobar</code> as the transaction identifier:
</p><pre class="programlisting">
PREPARE TRANSACTION 'foobar';
</pre></div><div class="refsect1" id="id-1.9.3.159.9"><h2>Compatibility</h2><p>
<code class="command">PREPARE TRANSACTION</code> is a
<span class="productname">PostgreSQL</span> 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.
</p></div><div class="refsect1" id="id-1.9.3.159.10"><h2>See Also</h2><span class="simplelist"><a class="xref" href="sql-commit-prepared.html" title="COMMIT PREPARED"><span class="refentrytitle">COMMIT PREPARED</span></a>, <a class="xref" href="sql-rollback-prepared.html" title="ROLLBACK PREPARED"><span class="refentrytitle">ROLLBACK PREPARED</span></a></span></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="sql-prepare.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-commands.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-reassign-owned.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">PREPARE </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> REASSIGN OWNED</td></tr></table></div></body></html>
