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
[ NOT ] DEFERRABLE

DESCRIPTION

Note

The following description applies both to Postgres-XC and PostgreSQL if not described explicitly.

BEGIN initiates a transaction block, that is, all statements after a BEGIN command will be executed in a single transaction until an explicit COMMIT(7) or ROLLBACK(7) is given. By default (without BEGIN), Postgres-XC 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 (SET_TRANSACTION(7)) was executed.

PARAMETERS

WORK, TRANSACTION

Optional key words. They have no effect.

Refer to SET TRANSACTION (SET_TRANSACTION(7)) for information on the meaning of the other parameters to this statement.

NOTES

START TRANSACTION (START_TRANSACTION(7)) has the same functionality as BEGIN.

Use COMMIT(7) or ROLLBACK(7) 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(7)).

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 Postgres-XC language extension. It is equivalent to the SQL-standard command START TRANSACTION (START_TRANSACTION(7)), whose reference page contains additional compatibility information.

The DEFERRABLEtransaction_mode is a Postgres-XC 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.