BEGIN
Name
BEGIN -- start a transaction block
Synopsis
BEGIN [ WORK | TRANSACTION ] [
transaction_mode
[, ...] ]
where
transaction_mode
is one of:
ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED }
READ WRITE | READ ONLY
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 or read/write 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
may 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
, which see for additional compatibility information.
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.
