BEGIN — start a transaction block
BEGIN [ WORK | TRANSACTION ] [transaction_mode
[, ...] ] wheretransaction_mode
is one of: ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED } READ WRITE | READ ONLY [ NOT ] DEFERRABLE
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.
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.
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.
To begin a transaction block:
BEGIN;
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.