BEGIN(l) SQL - Language Statements (2002-11-22) BEGIN(l)
NAME
BEGIN - start a transaction block
SYNOPSIS
BEGIN [ WORK | TRANSACTION ]
INPUTS
WORK
TRANSACTION
Optional keywords. They have no effect.
OUTPUTS
BEGIN
This signifies that a new transaction has been started.
WARNING: BEGIN: already a transaction in progress
This indicates that a transaction was already in
progress. The current transaction is not affected.
DESCRIPTION
By default, PostgreSQL executes transactions in unchained
mode (also known as ``autocommit'' in other database
systems). In other words, each user 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). BEGIN initiates a user
transaction in chained mode, i.e., all user statements after
BEGIN command will be executed in a single transaction until
an explicit COMMIT [commit(l)] or ROLLBACK [rollback(l)].
Statements are executed more quickly in chained mode,
because transaction start/commit requires significant CPU
and disk activity. Execution of multiple statements inside a
transaction is also useful to ensure consistency when
changing several related tables: other clients will be
unable to see the intermediate states wherein not all the
related updates have been done.
The default transaction isolation level in PostgreSQL is
READ COMMITTED, wherein each query inside the transaction
sees changes committed before that query begins execution.
So, you have to use SET TRANSACTION ISOLATION LEVEL
SERIALIZABLE just after BEGIN if you need more rigorous
transaction isolation. (Alternatively, you can change the
default transaction isolation level; see the PostgreSQL
Administrator's Guide for details.) In SERIALIZABLE mode
queries will see only changes committed before the entire
transaction began (actually, before execution of the first
DML statement in the transaction).
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BEGIN(l) SQL - Language Statements (2002-11-22) BEGIN(l)
Transactions have the standard ACID (atomic, consistent,
isolatable, and durable) properties.
NOTES
START TRANSACTION [start_transaction(l)] has the same
functionality as BEGIN.
Use COMMIT [commit(l)] or ROLLBACK [rollback(l)] to
terminate a transaction.
Refer to LOCK [lock(l)] for further information about
locking tables inside a transaction.
If you turn autocommit mode off, then BEGIN is not required:
any SQL command automatically starts a transaction.
USAGE
To begin a user transaction:
BEGIN WORK;
COMPATIBILITY
SQL92
BEGIN is a PostgreSQL language extension. There is no
explicit BEGIN command in SQL92; transaction initiation is
always implicit and it terminates either with a COMMIT or
ROLLBACK statement.
Note: Many relational database systems offer an
autocommit feature as a convenience.
Incidentally, the BEGIN keyword is used for a different
purpose in embedded SQL. You are advised to be careful about
the transaction semantics when porting database
applications.
SQL92 also requires SERIALIZABLE to be the default
transaction isolation level.
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