'\" t
.\"     Title: BEGIN
.\"    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 "BEGIN" "5sql" "2016" "PostgreSQL 9.5.2" "PostgreSQL 9.5.2 Documentation"
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.nh
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.SH "NAME"
BEGIN \- start a transaction block
.SH "SYNOPSIS"
.sp
.nf
BEGIN [ WORK | TRANSACTION ] [ \fItransaction_mode\fR [, \&.\&.\&.] ]

where \fItransaction_mode\fR is one of:

    ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED }
    READ WRITE | READ ONLY
    [ NOT ] DEFERRABLE
.fi
.SH "DESCRIPTION"
.PP

\fBBEGIN\fR
initiates a transaction block, that is, all statements after a
\fBBEGIN\fR
command will be executed in a single transaction until an explicit
\fBCOMMIT\fR(5)
or
\fBROLLBACK\fR(5)
is given\&. By default (without
\fBBEGIN\fR),
PostgreSQL
executes transactions in
\(lqautocommit\(rq
mode, that is, each statement is executed in its own transaction and a commit is implicitly performed at the end of the statement (if execution was successful, otherwise a rollback is done)\&.
.PP
Statements are executed more quickly in a transaction block, because transaction start/commit requires significant CPU and disk activity\&. Execution of multiple statements inside a transaction is also useful to ensure consistency when making several related changes: other sessions will be unable to see the intermediate states wherein not all the related updates have been done\&.
.PP
If the isolation level, read/write mode, or deferrable mode is specified, the new transaction has those characteristics, as if
SET TRANSACTION (\fBSET_TRANSACTION\fR(5))
was executed\&.
.SH "PARAMETERS"
.PP
WORK
.br
TRANSACTION
.RS 4
Optional key words\&. They have no effect\&.
.RE
.PP
Refer to
SET TRANSACTION (\fBSET_TRANSACTION\fR(5))
for information on the meaning of the other parameters to this statement\&.
.SH "NOTES"
.PP

START TRANSACTION (\fBSTART_TRANSACTION\fR(5))
has the same functionality as
\fBBEGIN\fR\&.
.PP
Use
\fBCOMMIT\fR(5)
or
\fBROLLBACK\fR(5)
to terminate a transaction block\&.
.PP
Issuing
\fBBEGIN\fR
when already inside a transaction block will provoke a warning message\&. The state of the transaction is not affected\&. To nest transactions within a transaction block, use savepoints (see
\fBSAVEPOINT\fR(5))\&.
.PP
For reasons of backwards compatibility, the commas between successive
\fItransaction_modes\fR
can be omitted\&.
.SH "EXAMPLES"
.PP
To begin a transaction block:
.sp
.if n \{\
.RS 4
.\}
.nf
BEGIN;
.fi
.if n \{\
.RE
.\}
.SH "COMPATIBILITY"
.PP

\fBBEGIN\fR
is a
PostgreSQL
language extension\&. It is equivalent to the SQL\-standard command
START TRANSACTION (\fBSTART_TRANSACTION\fR(5)), whose reference page contains additional compatibility information\&.
.PP
The
DEFERRABLE
\fItransaction_mode\fR
is a
PostgreSQL
language extension\&.
.PP
Incidentally, the
BEGIN
key word is used for a different purpose in embedded SQL\&. You are advised to be careful about the transaction semantics when porting database applications\&.
.SH "SEE ALSO"
\fBCOMMIT\fR(5), \fBROLLBACK\fR(5), START TRANSACTION (\fBSTART_TRANSACTION\fR(5)), \fBSAVEPOINT\fR(5)