begin(7) [redhat man page]

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)

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SET 
TRANSACTION(7)						   SQL Commands 						SET TRANSACTION(7)

NAME

       SET TRANSACTION - set the characteristics of the current transaction

SYNOPSIS

       SET TRANSACTION ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE }
       SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL
	   { READ COMMITTED | SERIALIZABLE }

DESCRIPTION

       This command sets the transaction isolation level. The SET TRANSACTION command sets the characteristics for the current SQL-transaction. It
       has no effect on any subsequent transactions. This command cannot be used after the first query	or  data-modification  statement  (SELECT,
       INSERT, DELETE, UPDATE, FETCH, COPY) of a transaction has been executed. SET SESSION CHARACTERISTICS sets the default transaction isolation
       level for each transaction for a session. SET TRANSACTION can override it for an individual transaction.

       The isolation level of a transaction determines what data the transaction can see when other transactions are running concurrently.

       READ COMMITTED
	      A statement can only see rows committed before it began. This is the default.

       SERIALIZABLE
	      The current transaction can only see rows committed before first query or data-modification statement was executed in this  transac-
	      tion.

	      Tip:  Intuitively,  serializable	means that two concurrent transactions will leave the database in the same state as if the two has
	      been executed strictly after one another in either order.

NOTES

       The session default transaction isolation level can also be set with the command

       SET default_transaction_isolation = 'value'

       and in the configuration file. Consult the Administrator's Guide for more information.

COMPATIBILITY

   SQL92, SQL99
       SERIALIZABLE is the default level in SQL. PostgreSQL does not provide the isolation levels READ UNCOMMITTED and REPEATABLE READ. Because of
       multiversion concurrency control, the SERIALIZABLE level is not truly serializable. See the User's Guide for details.

       In  SQL	there  are two other transaction characteristics that can be set with these commands: whether the transaction is read-only and the
       size of the diagnostics area.  Neither of these concepts are supported in PostgreSQL.

SQL - Language Statements					    2000-11-24							SET TRANSACTION(7)

Linux and UNIX Man Pages

begin(7) [redhat man page]

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