BEGIN(7)							   SQL Commands 							  BEGIN(7)

NAME

       BEGIN - start a transaction block

SYNOPSIS

       BEGIN [ WORK | TRANSACTION ]

   INPUTS
       WORK

       TRANSACTION
	      Optional keywords. They have no effect.

   OUTPUTS
       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.

DESCRIPTION

       By  default,  PostgreSQL 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
       successful,  otherwise a rollback is done).  BEGIN initiates a user transaction in chained mode, i.e., all user statements after BEGIN com-
       mand will be executed in a single transaction until an explicit COMMIT [commit(7)] or ROLLBACK [rollback(7)].  Statements 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 been done.

       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 rig-
       orous  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) properties.

   NOTES
       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.

USAGE

       To begin a user transaction:

       BEGIN WORK;

COMPATIBILITY

   SQL92
       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 ROLLBACK statement.

	      Note: Many relational database systems offer an autocommit feature 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 seman-
       tics when porting database applications.

       SQL92 also requires SERIALIZABLE to be the default transaction isolation level.

SQL - Language Statements					    2002-11-22								  BEGIN(7)