BEGIN() SQL Commands BEGIN()NAMEBEGIN - Begins a transaction in chained mode
SYNOPSISBEGIN [ WORK | TRANSACTION ]
INPUTS
WORK
TRANSACTION
Optional keywords. They have no effect.
OUTPUTS
BEGIN This signifies that a new transaction has been started.
NOTICE: BEGIN: already a transaction in progress
This indicates that a transaction was already in progress. The
current transaction is not affected.
DESCRIPTION
By default, Postgres executes transactions in unchained mode (also
known as ``autocommit'' in other database systems). In other words,
each user statement is executed in its own transaction and a commit is
implicitly performed at the end of the statement (if execution was suc‐
cessful, otherwise a rollback is done). BEGIN initiates a user trans‐
action in chained mode, i.e., all user statements after BEGIN command
will be executed in a single transaction until an explicit COMMIT [com‐
mit(l)], ROLLBACK [rollback(l)], or execution abort. Statements in
chained mode are executed much faster, because transaction start/commit
requires significant CPU and disk activity. Execution of multiple
statements inside a transaction is also required for consistency when
changing several related tables.
The default transaction isolation level in Postgres is READ COMMITTED,
where queries inside the transaction see only changes committed before
query execution. So, you have to use SET TRANSACTION ISOLATION LEVEL
SERIALIZABLE just after BEGIN if you need more rigorous transaction
isolation. In SERIALIZABLE mode queries will see only changes commit‐
ted before the entire transaction began (actually, before execution of
the first DML statement in a serializable transaction).
If the transaction is committed, Postgres will ensure either that all
updates are done or else that none of them are done. Transactions have
the standard ACID (atomic, consistent, isolatable, and durable) prop‐
erty.
NOTES
Refer to LOCK [lock(l)] for further information about locking tables
inside a transaction.
Use COMMIT [commit(l)] or ROLLBACK [rollback(l)] to terminate a trans‐
action.
USAGE
To begin a user transaction:
BEGIN WORK;
COMPATIBILITY
SQL92
BEGIN is a Postgres language extension. There is no explicit BEGIN
command in SQL92; transaction initiation is always implicit and it ter‐
minates either with a COMMIT or ROLLBACK statement.
Note: Many relational database systems offer an autocommit fea‐
ture as a convenience.
Incidentally, the BEGIN keyword is used for a different purpose in
embedded SQL. You are advised to be careful about the transaction
semantics when porting database applications.
SQL92 also requires SERIALIZABLE to be the default transaction isola‐
tion level.
SQL - Language Statements 29 March 2001 BEGIN()