PREPARE 
TRANSACTION(7)						   SQL Commands 					    PREPARE TRANSACTION(7)

NAME

       PREPARE TRANSACTION - prepare the current transaction for two-phase commit

SYNOPSIS

       PREPARE TRANSACTION transaction_id

DESCRIPTION

       PREPARE TRANSACTION prepares the current transaction for two-phase commit. After this command, the transaction is no longer associated with
       the current session; instead, its state is fully stored on disk, and there is a very high probability that it  can  be  committed  success-
       fully, even if a database crash occurs before the commit is requested.

       Once  prepared,	a  transaction can later be committed or rolled back with COMMIT PREPARED [commit_prepared(7)] or ROLLBACK PREPARED [roll-
       back_prepared(7)], respectively. Those commands can be issued from any session, not only the one that executed the original transaction.

       From the point of view of the issuing session, PREPARE TRANSACTION is not unlike a ROLLBACK command: after executing it, there is no active
       current	transaction,  and  the	effects  of  the prepared transaction are no longer visible. (The effects will become visible again if the
       transaction is committed.)

       If the PREPARE TRANSACTION command fails for any reason, it becomes a ROLLBACK: the current transaction is canceled.

PARAMETERS

       transaction_id
	      An arbitrary identifier that later identifies this transaction for COMMIT PREPARED or ROLLBACK PREPARED.	 The  identifier  must	be
	      written  as a string literal, and must be less than 200 bytes long. It must not be the same as the identifier used for any currently
	      prepared transaction.

NOTES

       This command must be used inside a transaction block. Use BEGIN [begin(7)] to start one.

       It is not currently allowed to PREPARE a transaction that has executed any operations involving temporary tables, created any cursors  WITH
       HOLD,  or executed LISTEN or UNLISTEN.  Those features are too tightly tied to the current session to be useful in a transaction to be pre-
       pared.

       If the transaction modified any run-time parameters with SET (without the LOCAL option), those effects persist after  PREPARE  TRANSACTION,
       and  will  not  be affected by any later COMMIT PREPARED or ROLLBACK PREPARED. Thus, in this one respect PREPARE TRANSACTION acts more like
       COMMIT than ROLLBACK.

       All currently available prepared transactions are listed in the pg_prepared_xacts system view.

	      Caution:

	      It is unwise to leave transactions in the prepared state for a long time.  This will interfere with the ability of VACUUM to reclaim
	      storage, and in extreme cases could cause the database to shut down to prevent transaction ID wraparound (see in the documentation).
	      Keep in mind also that the transaction continues to hold whatever locks it held. The intended usage of the feature is  that  a  pre-
	      pared transaction will normally be committed or rolled back as soon as an external transaction manager has verified that other data-
	      bases are also prepared to commit.

	      If you have not set up an external transaction manager to track prepared transactions and ensure they get closed out promptly, it is
	      best  to	keep  the prepared-transaction feature disabled by setting max_prepared_transactions to zero. This will prevent accidental
	      creation of prepared transactions that might then be forgotten and eventually cause problems.

EXAMPLES

       Prepare the current transaction for two-phase commit, using foobar as the transaction identifier:

       PREPARE TRANSACTION 'foobar';

SEE ALSO

       COMMIT PREPARED [commit_prepared(7)], ROLLBACK PREPARED [rollback_prepared(7)]

SQL - Language Statements					    2010-05-14						    PREPARE TRANSACTION(7)