BEGIN

Starts a transaction block.

Synopsis

BEGIN [WORK | TRANSACTION] [SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED]
      [READ WRITE | READ ONLY]

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), HAWQ 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.

Parameters

WORK
TRANSACTION

Optional key words. They have no effect.

SERIALIZABLE
REPEATABLE READ
READ COMMITTED
READ UNCOMMITTED

The SQL standard defines four transaction isolation levels: READ COMMITTED, READ UNCOMMITTED, SERIALIZABLE, and REPEATABLE READ. The default behavior is that a statement can only see rows committed before it began (READ COMMITTED). In HAWQ, READ UNCOMMITTED is treated the same as READ COMMITTED. SERIALIZABLE is supported the same as REPEATABLE READ wherein all statements of the current transaction can only see rows committed before the first statement was executed in the transaction. SERIALIZABLE is the strictest transaction isolation. This level emulates serial transaction execution, as if transactions had been executed one after another, serially, rather than concurrently. Applications using this level must be prepared to retry transactions due to serialization failures.

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 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 execute is among those listed.

Notes

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;

Compatibility

BEGIN is a HAWQ language extension. It is equivalent to the SQL-standard command START TRANSACTION lang="EN".

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, SAVEPOINT