1<!--
2doc/src/sgml/ref/prepare_transaction.sgml
3PostgreSQL documentation
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5
6<refentry id="sql-prepare-transaction">
7 <indexterm zone="sql-prepare-transaction">
8  <primary>PREPARE TRANSACTION</primary>
9 </indexterm>
10
11 <refmeta>
12  <refentrytitle>PREPARE TRANSACTION</refentrytitle>
13  <manvolnum>7</manvolnum>
14  <refmiscinfo>SQL - Language Statements</refmiscinfo>
15 </refmeta>
16
17 <refnamediv>
18  <refname>PREPARE TRANSACTION</refname>
19  <refpurpose>prepare the current transaction for two-phase commit</refpurpose>
20 </refnamediv>
21
22 <refsynopsisdiv>
23<synopsis>
24PREPARE TRANSACTION <replaceable class="parameter">transaction_id</replaceable>
25</synopsis>
26 </refsynopsisdiv>
27
28 <refsect1>
29  <title>Description</title>
30
31  <para>
32   <command>PREPARE TRANSACTION</command> prepares the current transaction
33   for two-phase commit. After this command, the transaction is no longer
34   associated with the current session; instead, its state is fully stored on
35   disk, and there is a very high probability that it can be committed
36   successfully, even if a database crash occurs before the commit is
37   requested.
38  </para>
39
40  <para>
41   Once prepared, a transaction can later be committed or rolled back
42   with <xref linkend="sql-commit-prepared"/>
43   or <xref linkend="sql-rollback-prepared"/>,
44   respectively.  Those commands can be issued from any session, not
45   only the one that executed the original transaction.
46  </para>
47
48  <para>
49   From the point of view of the issuing session, <command>PREPARE
50   TRANSACTION</command> is not unlike a <command>ROLLBACK</command> command:
51   after executing it, there is no active current transaction, and the
52   effects of the prepared transaction are no longer visible.  (The effects
53   will become visible again if the transaction is committed.)
54  </para>
55
56  <para>
57   If the <command>PREPARE TRANSACTION</command> command fails for any
58   reason, it becomes a <command>ROLLBACK</command>: the current transaction
59   is canceled.
60  </para>
61 </refsect1>
62
63 <refsect1>
64  <title>Parameters</title>
65
66  <variablelist>
67   <varlistentry>
68    <term><replaceable class="parameter">transaction_id</replaceable></term>
69    <listitem>
70     <para>
71      An arbitrary identifier that later identifies this transaction for
72      <command>COMMIT PREPARED</command> or <command>ROLLBACK PREPARED</command>.
73      The identifier must be written as a string literal, and must be
74      less than 200 bytes long.  It must not be the same as the identifier
75      used for any currently prepared transaction.
76     </para>
77    </listitem>
78   </varlistentry>
79  </variablelist>
80 </refsect1>
81
82 <refsect1>
83  <title>Notes</title>
84
85  <para>
86   <command>PREPARE TRANSACTION</command> is not intended for use in applications
87   or interactive sessions. Its purpose is to allow an external
88   transaction manager to perform atomic global transactions across multiple
89   databases or other transactional resources. Unless you're writing a
90   transaction manager, you probably shouldn't be using <command>PREPARE
91   TRANSACTION</command>.
92  </para>
93
94  <para>
95   This command must be used inside a transaction block. Use <xref
96   linkend="sql-begin"/> to start one.
97  </para>
98
99  <para>
100   It is not currently allowed to <command>PREPARE</command> a transaction that
101   has executed any operations involving temporary tables or the session's
102   temporary namespace, created any cursors <literal>WITH HOLD</literal>, or
103   executed <command>LISTEN</command>, <command>UNLISTEN</command>, or
104   <command>NOTIFY</command>.
105   Those features are too tightly
106   tied to the current session to be useful in a transaction to be prepared.
107  </para>
108
109  <para>
110   If the transaction modified any run-time parameters with <command>SET</command>
111   (without the <literal>LOCAL</literal> option),
112   those effects persist after <command>PREPARE TRANSACTION</command>, and will not
113   be affected by any later <command>COMMIT PREPARED</command> or
114   <command>ROLLBACK PREPARED</command>.  Thus, in this one respect
115   <command>PREPARE TRANSACTION</command> acts more like <command>COMMIT</command> than
116   <command>ROLLBACK</command>.
117  </para>
118
119  <para>
120   All currently available prepared transactions are listed in the
121   <link linkend="view-pg-prepared-xacts"><structname>pg_prepared_xacts</structname></link>
122   system view.
123  </para>
124
125  <caution>
126   <para>
127    It is unwise to leave transactions in the prepared state for a long time.
128    This will interfere with the ability of <command>VACUUM</command> to reclaim
129    storage, and in extreme cases could cause the database to shut down
130    to prevent transaction ID wraparound (see <xref
131    linkend="vacuum-for-wraparound"/>).  Keep in mind also that the transaction
132    continues to hold whatever locks it held.  The intended usage of the
133    feature is that a prepared transaction will normally be committed or
134    rolled back as soon as an external transaction manager has verified that
135    other databases are also prepared to commit.
136   </para>
137
138   <para>
139    If you have not set up an external transaction manager to track prepared
140    transactions and ensure they get closed out promptly, it is best to keep
141    the prepared-transaction feature disabled by setting
142    <xref linkend="guc-max-prepared-transactions"/> to zero.  This will
143    prevent accidental creation of prepared transactions that might then
144    be forgotten and eventually cause problems.
145   </para>
146  </caution>
147 </refsect1>
148
149 <refsect1 id="sql-prepare-transaction-examples">
150  <title>Examples</title>
151  <para>
152   Prepare the current transaction for two-phase commit, using
153   <literal>foobar</literal> as the transaction identifier:
154
155<programlisting>
156PREPARE TRANSACTION 'foobar';
157</programlisting></para>
158 </refsect1>
159
160 <refsect1>
161  <title>Compatibility</title>
162
163  <para>
164   <command>PREPARE TRANSACTION</command> is a
165   <productname>PostgreSQL</productname> extension.  It is intended for use by
166   external transaction management systems, some of which are covered by
167   standards (such as X/Open XA), but the SQL side of those systems is not
168   standardized.
169  </para>
170 </refsect1>
171
172 <refsect1>
173  <title>See Also</title>
174
175  <simplelist type="inline">
176   <member><xref linkend="sql-commit-prepared"/></member>
177   <member><xref linkend="sql-rollback-prepared"/></member>
178  </simplelist>
179 </refsect1>
180
181</refentry>
182