BEGIN SQL - Language Statements BEGIN start a transaction block BEGIN [ WORK | TRANSACTION ] By default, PostgreSQL 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). BEGIN initiates a transaction block, that is, all statements after BEGIN command will be executed in a single transaction until an explicit or . 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 changing several related tables: other sessions will be unable to see the intermediate states wherein not all the related updates have been done. WORK TRANSACTION Optional key words. 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. has the same functionality as BEGIN. Use or to terminate a transaction. If you turn the configuration parameter autocommit off, then BEGIN is not required: any SQL command automatically starts a transaction. To begin a transaction block: BEGIN; BEGIN is a PostgreSQL language extension. There is no explicit BEGIN command in the SQL standard; transaction initiation is always implicit and it terminates either with a COMMIT or ROLLBACK statement. Other relational database systems may offer an autocommit feature as a convenience. 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.