## BEGIN

BEGIN — start a transaction block

## Synopsis

    BEGIN [ AUTONOMOUS ] [ 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

`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`), Postgres Pro 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.

In case of presence of optional `AUTONOMOUS` keyword, autonomous transaction is started. Autonomous transaction can be started only within some other transaction. See section Chapter 16 for more details about autonomous transactions.

## Parameters

`WORK`
`TRANSACTION`

  Optional key words. They have no effect.

`AUTONOMOUS`

  Optional key word. Cause start of autonomous subtransaction. Autonomous transaction can be started only within some other transaction. See section Chapter 16 for more details about autonomous transactions.

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` can be omitted.

## Examples

To begin a transaction block:

    BEGIN;

## Compatibility

`BEGIN` is a Postgres Pro 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 Postgres Pro 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