BEGIN [ WORK | TRANSACTION ] [ transaction_mode [, ...] ]
where transaction_mode is one of:
ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED }
READ WRITE | READ ONLY
[ NOT ] DEFERRABLEBEGIN 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.
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.
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.