BEGIN command to initiate a transaction block - PolarDB

BEGIN starts a transaction block. All statements after BEGIN execute as a single transaction until you issue an explicit COMMIT or ROLLBACK.

Without BEGIN, PolarDB PostgreSQL runs in autocommit mode: each statement executes in its own transaction and commits automatically on success, or rolls back on failure.

Why use a transaction block

Grouping statements in a transaction block offers two advantages:

Performance: Starting and committing a transaction requires significant CPU and disk activity. Batching multiple statements into one transaction reduces that overhead.
Consistency: Other sessions cannot see intermediate states. Until you commit, any partially applied changes are invisible to other connections.

If the isolation level, read/write mode, or deferrable mode is specified, the new transaction has those characteristics, as if SET TRANSACTION was executed.

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

Parameters

Parameter	Description
`WORK` \| `TRANSACTION`	Optional keywords. They have no effect on behavior.
`ISOLATION LEVEL`	Sets the transaction isolation level. Valid values: `SERIALIZABLE`, `REPEATABLE READ`, `READ COMMITTED`, `READ UNCOMMITTED`.
`READ WRITE` \| `READ ONLY`	Sets the transaction access mode. `READ ONLY` prevents data modifications in the transaction.
`DEFERRABLE` \| `NOT DEFERRABLE`	Controls whether a `SERIALIZABLE READ ONLY` transaction can be deferred to avoid blocking. Has no effect on other transaction types.

For full details on each transaction mode, see SET TRANSACTION.

Usage notes

SET TRANSACTION has the same functionality as BEGIN.
To end a transaction block, use COMMIT or ROLLBACK.
Issuing BEGIN inside an existing transaction block triggers a warning. The current transaction state is not affected.
To nest transactions within a transaction block, use savepoints. See SAVEPOINT.
For backward compatibility, you can omit commas between successive transaction_mode values.

Examples

Start a transaction with default settings

BEGIN;

Set the isolation level

BEGIN TRANSACTION ISOLATION LEVEL READ COMMITTED;

Start a read-only transaction

BEGIN READ ONLY;

Combine isolation level and access mode

BEGIN ISOLATION LEVEL SERIALIZABLE READ ONLY DEFERRABLE;

PolarDB:BEGIN