START TRANSACTION

Starts a transaction block.

Synopsis

START TRANSACTION [<transaction_mode>] [READ WRITE | READ ONLY]

where transaction_mode is:

   ISOLATION LEVEL {SERIALIZABLE | READ COMMITTED | READ UNCOMMITTED}

Description

START TRANSACTION begins a new transaction block. If the isolation level or read/write mode is specified, the new transaction has those characteristics, as if SET TRANSACTION was run. This is the same as the BEGIN command.

Parameters

READ UNCOMMITTED

READ COMMITTED

REPEATABLE READ

SERIALIZABLE

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 Greenplum Database, so READ UNCOMMITTED is treated the same as READ COMMITTED.

READ COMMITTED, the default isolation level in Greenplum Database, 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 Greenplum Database 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, Greenplum Database falls back to REPEATABLE READ.

READ WRITE

READ ONLY

Determines whether the transaction is read/write or read-only. Read/write is the default. When a transaction is read-only, the following SQL commands are disallowed: INSERT, UPDATE, DELETE, and COPY FROM if the table they would write to is not a temporary table; all CREATE, ALTER, and DROP commands; GRANT, REVOKE, TRUNCATE; and EXPLAIN ANALYZE and EXECUTE if the command they would run is among those listed.

Examples

To begin a transaction block:

START TRANSACTION;

Compatibility

In the standard, it is not necessary to issue START TRANSACTION to start a transaction block: any SQL command implicitly begins a block. Greenplum Database behavior can be seen as implicitly issuing a COMMIT after each command that does not follow START TRANSACTION (or BEGIN), and it is therefore often called 'autocommit'. Other relational database systems may offer an autocommit feature as a convenience.

The SQL standard requires commas between successive transaction_modes, but for historical reasons Greenplum Database allows the commas to be omitted.

See also the compatibility section of SET TRANSACTION.

See Also

BEGIN, SET TRANSACTION

Parent topic: SQL Commands