'\" t
.\"     Title: PREPARE TRANSACTION
.\"    Author: The PostgreSQL Global Development Group
.\" Generator: DocBook XSL Stylesheets v1.75.2 <http://docbook.sf.net/>
.\"      Date: 2016
.\"    Manual: PostgreSQL 9.5.2 Documentation
.\"    Source: PostgreSQL 9.5.2
.\"  Language: English
.\"
.TH "PREPARE TRANSACTION" "5sql" "2016" "PostgreSQL 9.5.2" "PostgreSQL 9.5.2 Documentation"
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.SH "NAME"
PREPARE_TRANSACTION \- prepare the current transaction for two\-phase commit
.SH "SYNOPSIS"
.sp
.nf
PREPARE TRANSACTION \fItransaction_id\fR
.fi
.SH "DESCRIPTION"
.PP

\fBPREPARE TRANSACTION\fR
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\&.
.PP
Once prepared, a transaction can later be committed or rolled back with
COMMIT PREPARED (\fBCOMMIT_PREPARED\fR(5))
or
ROLLBACK PREPARED (\fBROLLBACK_PREPARED\fR(5)), respectively\&. Those commands can be issued from any session, not only the one that executed the original transaction\&.
.PP
From the point of view of the issuing session,
\fBPREPARE TRANSACTION\fR
is not unlike a
\fBROLLBACK\fR
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\&.)
.PP
If the
\fBPREPARE TRANSACTION\fR
command fails for any reason, it becomes a
\fBROLLBACK\fR: the current transaction is canceled\&.
.SH "PARAMETERS"
.PP
\fItransaction_id\fR
.RS 4
An arbitrary identifier that later identifies this transaction for
\fBCOMMIT PREPARED\fR
or
\fBROLLBACK PREPARED\fR\&. 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\&.
.RE
.SH "NOTES"
.PP

\fBPREPARE TRANSACTION\fR
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\(aqre writing a transaction manager, you probably shouldn\(aqt be using
\fBPREPARE TRANSACTION\fR\&.
.PP
This command must be used inside a transaction block\&. Use
\fBBEGIN\fR(5)
to start one\&.
.PP
It is not currently allowed to
\fBPREPARE\fR
a transaction that has executed any operations involving temporary tables, created any cursors
WITH HOLD, or executed
\fBLISTEN\fR
or
\fBUNLISTEN\fR\&. Those features are too tightly tied to the current session to be useful in a transaction to be prepared\&.
.PP
If the transaction modified any run\-time parameters with
\fBSET\fR
(without the
LOCAL
option), those effects persist after
\fBPREPARE TRANSACTION\fR, and will not be affected by any later
\fBCOMMIT PREPARED\fR
or
\fBROLLBACK PREPARED\fR\&. Thus, in this one respect
\fBPREPARE TRANSACTION\fR
acts more like
\fBCOMMIT\fR
than
\fBROLLBACK\fR\&.
.PP
All currently available prepared transactions are listed in the
pg_prepared_xacts
system view\&.
.if n \{\
.sp
.\}
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
\fBCaution\fR
.ps -1
.br
.PP
It is unwise to leave transactions in the prepared state for a long time\&. This will interfere with the ability of
\fBVACUUM\fR
to reclaim storage, and in extreme cases could cause the database to shut down to prevent transaction ID wraparound (see
Section 23.1.5, \(lqPreventing Transaction ID Wraparound Failures\(rq, in the documentation)\&. 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\&.
.PP
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
max_prepared_transactions
to zero\&. This will prevent accidental creation of prepared transactions that might then be forgotten and eventually cause problems\&.
.sp .5v
.RE
.SH "EXAMPLES"
.PP
Prepare the current transaction for two\-phase commit, using
foobar
as the transaction identifier:
.sp
.if n \{\
.RS 4
.\}
.nf
PREPARE TRANSACTION \(aqfoobar\(aq;
.fi
.if n \{\
.RE
.\}
.SH "COMPATIBILITY"
.PP

\fBPREPARE TRANSACTION\fR
is a
PostgreSQL
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\&.
.SH "SEE ALSO"
COMMIT PREPARED (\fBCOMMIT_PREPARED\fR(5)), ROLLBACK PREPARED (\fBROLLBACK_PREPARED\fR(5))