Starts a transaction block.
BEGIN [WORK | TRANSACTION] [<transaction_mode>]
[READ ONLY | READ WRITE]
where transaction_mode is one of:
ISOLATION LEVEL | {SERIALIZABLE | READ COMMITTED | READ UNCOMMITTED}
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
), Greenplum Database 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 or read/write mode is specified, the new transaction has those characteristics, as if SET TRANSACTION was executed.
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 Greenplum Database
READ UNCOMMITTED
is treated the same as
READ COMMITTED
.
REPEATABLE READ
is not supported; use
SERIALIZABLE
if this behavior is required.
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.
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 execute is among those listed.
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
).
To begin a transaction block:
BEGIN;
To begin a transaction block with the serializable isolation level:
BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
BEGIN
is a Greenplum Database language extension. It is equivalent to the SQL-standard command START TRANSACTION.
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.
COMMIT, ROLLBACK, START TRANSACTION, SAVEPOINT
Parent topic: SQL Command Reference