BEGIN v6.27.4

Starts a transaction block.

Synopsis

BEGIN [WORK | TRANSACTION] [<transaction_mode>]

where transaction_mode is:

   ISOLATION LEVEL {READ UNCOMMITTED | READ COMMITTED | REPEATABLE READ | SERIALIZABLE}
   READ WRITE | READ ONLY
   [ NOT ] DEFERRABLE

Description

BEGIN initiates a transaction block, that is, all statements after a BEGIN command will be run in a single transaction until an explicit COMMIT or ROLLBACK is given. By default (without BEGIN), WarehousePG runs transactions in autocommit mode, that is, each statement is run 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 run 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 run.

Parameters

• WORK

• TRANSACTION

  Optional key words. They have no effect.

• SERIALIZABLE

• READ COMMITTED

• READ UNCOMMITTED

  The SQL standard defines four transaction isolation levels: READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, and SERIALIZABLE.

  READ UNCOMMITTED allows transactions to see changes made by uncomitted concurrent transactions. This is not possible in WarehousePG, so READ UNCOMMITTED is treated the same as READ COMMITTED.

  READ COMMITTED, the default isolation level in WarehousePG, guarantees that a statement can only see rows committed before it began. The same statement run twice in a transaction can produce different results if another concurrent transaction commits after the statement is run the first time.

  The REPEATABLE READ isolation level guarantees that a transaction can only see rows committed before it began. REPEATABLE READ is the strictest transaction isolation level WarehousePG supports. Applications that use the REPEATABLE READ isolation level must be prepared to retry transactions due to serialization failures.

  The SERIALIZABLE transaction isolation level guarantees that running multiple concurrent transactions produces the same effects as running the same transactions one at a time. If you specify SERIALIZABLE, WarehousePG falls back to REPEATABLE READ.

  Specifying DEFERRABLE has no effect in WarehousePG, but the syntax is supported for compatibility with PostgreSQL. A transaction can only be deferred if it is READ ONLY and SERIALIZABLE, and WarehousePG does not support SERIALIAZABLE transactions.

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).

Examples

To begin a transaction block:

BEGIN;

To begin a transaction block with the repeatable read isolation level:

BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;

Compatibility

BEGIN is a WarehousePG language extension. It is equivalent to the SQL-standard command START TRANSACTION.

DEFERRABLE transaction_mode is a WarehousePG 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.

See Also

COMMIT, ROLLBACK, START TRANSACTION, SAVEPOINT

Parent topic: SQL Commands