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Green_GoodERP18_FSJ_Standard/PostgreSQL/doc/postgresql/html/sql-prepare-transaction.html

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  1. <?xml version="1.0" encoding="UTF-8" standalone="no"?>

  2. <!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 V1.79.1" /><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 12.4 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">

  3. PREPARE TRANSACTION <em class="replaceable"><code>transaction_id</code></em>

  4. </pre></div><div class="refsect1" id="id-1.9.3.159.5"><h2>Description</h2><p>

  5.    <code class="command">PREPARE TRANSACTION</code> prepares the current transaction

  6.    for two-phase commit. After this command, the transaction is no longer

  7.    associated with the current session; instead, its state is fully stored on

  8.    disk, and there is a very high probability that it can be committed

  9.    successfully, even if a database crash occurs before the commit is

  10.    requested.

  11.   </p><p>

  12.    Once prepared, a transaction can later be committed or rolled back

  13.    with <a class="xref" href="sql-commit-prepared.html" title="COMMIT PREPARED"><span class="refentrytitle">COMMIT PREPARED</span></a>

  14.    or <a class="xref" href="sql-rollback-prepared.html" title="ROLLBACK PREPARED"><span class="refentrytitle">ROLLBACK PREPARED</span></a>,

  15.    respectively.  Those commands can be issued from any session, not

  16.    only the one that executed the original transaction.

  17.   </p><p>

  18.    From the point of view of the issuing session, <code class="command">PREPARE

  19.    TRANSACTION</code> is not unlike a <code class="command">ROLLBACK</code> command:

  20.    after executing it, there is no active current transaction, and the

  21.    effects of the prepared transaction are no longer visible.  (The effects

  22.    will become visible again if the transaction is committed.)

  23.   </p><p>

  24.    If the <code class="command">PREPARE TRANSACTION</code> command fails for any

  25.    reason, it becomes a <code class="command">ROLLBACK</code>: the current transaction

  26.    is canceled.

  27.   </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>

  28.       An arbitrary identifier that later identifies this transaction for

  29.       <code class="command">COMMIT PREPARED</code> or <code class="command">ROLLBACK PREPARED</code>.

  30.       The identifier must be written as a string literal, and must be

  31.       less than 200 bytes long.  It must not be the same as the identifier

  32.       used for any currently prepared transaction.

  33.      </p></dd></dl></div></div><div class="refsect1" id="id-1.9.3.159.7"><h2>Notes</h2><p>

  34.    <code class="command">PREPARE TRANSACTION</code> is not intended for use in applications

  35.    or interactive sessions. Its purpose is to allow an external

  36.    transaction manager to perform atomic global transactions across multiple

  37.    databases or other transactional resources. Unless you're writing a

  38.    transaction manager, you probably shouldn't be using <code class="command">PREPARE

  39.    TRANSACTION</code>.

  40.   </p><p>

  41.    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.

  42.   </p><p>

  43.    It is not currently allowed to <code class="command">PREPARE</code> a transaction that

  44.    has executed any operations involving temporary tables or the session's

  45.    temporary namespace, created any cursors <code class="literal">WITH HOLD</code>, or

  46.    executed <code class="command">LISTEN</code>, <code class="command">UNLISTEN</code>, or

  47.    <code class="command">NOTIFY</code>.

  48.    Those features are too tightly

  49.    tied to the current session to be useful in a transaction to be prepared.

  50.   </p><p>

  51.    If the transaction modified any run-time parameters with <code class="command">SET</code>

  52.    (without the <code class="literal">LOCAL</code> option),

  53.    those effects persist after <code class="command">PREPARE TRANSACTION</code>, and will not

  54.    be affected by any later <code class="command">COMMIT PREPARED</code> or

  55.    <code class="command">ROLLBACK PREPARED</code>.  Thus, in this one respect

  56.    <code class="command">PREPARE TRANSACTION</code> acts more like <code class="command">COMMIT</code> than

  57.    <code class="command">ROLLBACK</code>.

  58.   </p><p>

  59.    All currently available prepared transactions are listed in the

  60.    <a class="link" href="view-pg-prepared-xacts.html" title="51.78. pg_prepared_xacts"><code class="structname">pg_prepared_xacts</code></a>

  61.    system view.

  62.   </p><div class="caution"><h3 class="title">Caution</h3><p>

  63.     It is unwise to leave transactions in the prepared state for a long time.

  64.     This will interfere with the ability of <code class="command">VACUUM</code> to reclaim

  65.     storage, and in extreme cases could cause the database to shut down

  66.     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

  67.     continues to hold whatever locks it held.  The intended usage of the

  68.     feature is that a prepared transaction will normally be committed or

  69.     rolled back as soon as an external transaction manager has verified that

  70.     other databases are also prepared to commit.

  71.    </p><p>

  72.     If you have not set up an external transaction manager to track prepared

  73.     transactions and ensure they get closed out promptly, it is best to keep

  74.     the prepared-transaction feature disabled by setting

  75.     <a class="xref" href="runtime-config-resource.html#GUC-MAX-PREPARED-TRANSACTIONS">max_prepared_transactions</a> to zero.  This will

  76.     prevent accidental creation of prepared transactions that might then

  77.     be forgotten and eventually cause problems.

  78.    </p></div></div><div class="refsect1" id="SQL-PREPARE-TRANSACTION-EXAMPLES"><h2>Examples</h2><p>

  79.    Prepare the current transaction for two-phase commit, using

  80.    <code class="literal">foobar</code> as the transaction identifier:

  81. 

  82. </p><pre class="programlisting">

  83. PREPARE TRANSACTION 'foobar';

  84. </pre></div><div class="refsect1" id="id-1.9.3.159.9"><h2>Compatibility</h2><p>

  85.    <code class="command">PREPARE TRANSACTION</code> is a

  86.    <span class="productname">PostgreSQL</span> extension.  It is intended for use by

  87.    external transaction management systems, some of which are covered by

  88.    standards (such as X/Open XA), but the SQL side of those systems is not

  89.    standardized.

  90.   </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>
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