Starts a transaction block.
BEGIN [WORK | TRANSACTION] [SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED] [READ WRITE | READ ONLY]
BEGIN initiates a transaction block, that is, all statements after a
BEGIN command will be executed in a single transaction until an explicit
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.
Optional key words. They have no effect.
The SQL standard defines four transaction isolation levels:
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
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.
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:
COPY FROM if the table they would write to is not a temporary table; all
EXPLAIN ANALYZE and
EXECUTE if the command they would execute is among those listed.
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).
To begin a transaction block:
BEGIN is a HAWQ language extension. It is equivalent to the SQL-standard command
START TRANSACTION lang="EN".
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.