BEGIN
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## BEGIN

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
        [ 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`), 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.

## 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` can 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`, 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.

## See Also

COMMIT, ROLLBACK, START TRANSACTION, SAVEPOINT

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