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<a name="sql-prepare-transaction"></a><div class="titlepage"></div>
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<div class="refnamediv">
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<p>PREPARE TRANSACTION — prepare the current transaction for two-phase commit</p>
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<a name="id779697"></a><div class="refsynopsisdiv">
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<pre class="synopsis">PREPARE TRANSACTION <em class="replaceable"><code>transaction_id</code></em></pre>
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<div class="refsect1" lang="en">
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<a name="id779719"></a><h2>Description</h2>
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<p> <code class="command">PREPARE TRANSACTION</code> prepares the current transaction
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for two-phase commit. After this command, the transaction is no longer
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associated with the current session; instead, its state is fully stored on
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disk, and there is a very high probability that it can be committed
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successfully, even if a database crash occurs before the commit is
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<p> Once prepared, a transaction can later be committed or rolled
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back with <code class="command">COMMIT PREPARED</code> or
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<code class="command">ROLLBACK PREPARED</code>, respectively. Those commands
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can be issued from any session, not only the one that executed the
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<p> From the point of view of the issuing session, <code class="command">PREPARE
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TRANSACTION</code> is not unlike a <code class="command">ROLLBACK</code> command:
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after executing it, there is no active current transaction, and the
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effects of the prepared transaction are no longer visible. (The effects
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will become visible again if the transaction is committed.)
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<p> If the <code class="command">PREPARE TRANSACTION</code> command fails for any
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reason, it becomes a <code class="command">ROLLBACK</code>: the current transaction
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<div class="refsect1" lang="en">
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<a name="id779796"></a><h2>Parameters</h2>
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<div class="variablelist"><dl>
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<dt><span class="term"><em class="replaceable"><code>transaction_id</code></em></span></dt>
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<dd><p> An arbitrary identifier that later identifies this transaction for
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<code class="command">COMMIT PREPARED</code> or <code class="command">ROLLBACK PREPARED</code>.
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The identifier must be written as a string literal, and must be
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less than 200 bytes long. It must not be the same as the identifier
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used for any currently prepared transaction.
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<div class="refsect1" lang="en">
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<a name="id779831"></a><h2>Notes</h2>
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<p> This command must be used inside a transaction block. Use
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<code class="command">BEGIN</code> to start one.
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<p> It is not currently allowed to <code class="command">PREPARE</code> a transaction that
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has executed any operations involving temporary tables or
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created any cursors <code class="literal">WITH HOLD</code>. Those features are too tightly
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tied to the current session to be useful in a transaction to be prepared.
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<p> If the transaction modified any run-time parameters with <code class="command">SET</code>,
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those effects persist after <code class="command">PREPARE TRANSACTION</code>, and will not
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be affected by any later <code class="command">COMMIT PREPARED</code> or
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<code class="command">ROLLBACK PREPARED</code>. Thus, in this one respect
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<code class="command">PREPARE TRANSACTION</code> acts more like <code class="command">COMMIT</code> than
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<code class="command">ROLLBACK</code>.
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<p> All currently available prepared transactions are listed in the
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<code class="structname">pg_prepared_xacts</code> system view.
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<p> From a performance standpoint, it is unwise to leave transactions in
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the prepared state for a long time: this will for instance interfere with
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the ability of <code class="command">VACUUM</code> to reclaim storage. Keep in mind also
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that the transaction continues to hold whatever locks it held.
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usage of the feature is that a prepared transaction will normally be
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committed or rolled back as soon as an external transaction manager
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has verified that other databases are also prepared to commit.
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<p> If you make any serious use of prepared transactions, you will probably
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want to increase the value of <a href="runtime-config-resource.html#guc-max-prepared-transactions">max_prepared_transactions</a>, as the default setting is
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quite small (to avoid wasting resources for those who don't use it).
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It is recommendable to make it at least equal to
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<a href="runtime-config-connection.html#guc-max-connections">max_connections</a>, so that every session can have
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a prepared transaction pending.
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<div class="refsect1" lang="en">
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<a name="sql-prepare-transaction-examples"></a><h2>Examples</h2>
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<p> Prepare the current transaction for two-phase commit, using
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<code class="literal">foobar</code> as the transaction identifier:
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<pre class="programlisting">PREPARE TRANSACTION 'foobar';</pre>
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<div class="refsect1" lang="en">
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<a name="id779997"></a><h2>See Also</h2>
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<span class="simplelist"><a href="sql-commit-prepared.html">COMMIT PREPARED</a>, <a href="sql-rollback-prepared.html">ROLLBACK PREPARED</a></span>
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