BEGIN
BEGIN
BEGIN - start a transaction block
Synopsis
BEGIN [ WORK | TRANSACTION ] [transaction_mode[, ...] ] wheretransaction_modeis one of: ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED } READ WRITE | READ ONLY [ NOT ] DEFERRABLE
Description
BEGIN
initiates a transaction block, that is,
all statements after a
BEGIN
command will be
executed in a single transaction until an explicit
COMMIT
or
ROLLBACK
is given.
By default (without
BEGIN
),
PostgreSQL
executes
transactions in
"
autocommit
"
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).
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.
If the isolation level, read/write mode, or deferrable mode is specified, the new
transaction has those characteristics, as if
SET TRANSACTION
was executed.
Parameters
-
WORK
TRANSACTION -
Optional key words. They have no effect.
Refer to SET TRANSACTION for information on the meaning of the other parameters to this statement.
Notes
START TRANSACTION
has the same functionality
as
BEGIN
.
Use
COMMIT
or
ROLLBACK
to terminate a transaction block.
Issuing
BEGIN
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
SAVEPOINT
).
For reasons of backwards compatibility, the commas between successive
transaction_modes
can be
omitted.
Examples
To begin a transaction block:
BEGIN;
Compatibility
BEGIN
is a
PostgreSQL
language extension. It is equivalent to the SQL-standard command
START TRANSACTION
, whose reference page
contains additional compatibility information.
The
DEFERRABLE
transaction_mode
is a
PostgreSQL
language extension.
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.