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BEGIN(7) SQL Commands BEGIN(7)
BEGIN - start a transaction block
BEGIN [ WORK | TRANSACTION ]
Optional keywords. They have no effect.
BEGIN This signifies that a new transaction has been started.
WARNING: BEGIN: already a transaction in progress
This indicates that a transaction was already in progress. The current transaction
is not affected.
By default, PostgreSQL executes transactions in unchained mode (also known as ``autocom-
mit'' 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 exe-
cution was successful, otherwise a rollback is done). BEGIN initiates a user transaction
in chained mode, i.e., all user statements after BEGIN command will be executed in a sin-
gle transaction until an explicit COMMIT [commit(7)] or ROLLBACK [rollback(7)]. State-
ments are executed more quickly in chained mode, because transaction start/commit requires
significant CPU and disk activity. Execution of multiple statements inside a transaction
is also useful to ensure consistency when changing several related tables: other clients
will be unable to see the intermediate states wherein not all the related updates have
The default transaction isolation level in PostgreSQL is READ COMMITTED, wherein each
query inside the transaction sees changes committed before that query begins execution.
So, you have to use SET TRANSACTION ISOLATION LEVEL SERIALIZABLE just after BEGIN if you
need more rigorous transaction isolation. (Alternatively, you can change the default
transaction isolation level; see the PostgreSQL Administrator's Guide for details.) In
SERIALIZABLE mode queries will see only changes committed before the entire transaction
began (actually, before execution of the first DML statement in the transaction).
Transactions have the standard ACID (atomic, consistent, isolatable, and durable) proper-
START TRANSACTION [start_transaction(7)] has the same functionality as BEGIN.
Use COMMIT [commit(7)] or ROLLBACK [rollback(7)] to terminate a transaction.
Refer to LOCK [lock(7)] for further information about locking tables inside a transaction.
If you turn autocommit mode off, then BEGIN is not required: any SQL command automatically
starts a transaction.
To begin a user transaction:
BEGIN is a PostgreSQL language extension. There is no explicit BEGIN command in SQL92;
transaction initiation is always implicit and it terminates either with a COMMIT or ROLL-
Note: Many relational database systems offer an autocommit feature as a conve-
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 isolation level.
SQL - Language Statements 2002-11-22 BEGIN(7)
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