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PREPARE	TRANSACTION(7)	PostgreSQL 9.6.3 Documentation	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
       successfully, 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
       (ROLLBACK_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
       PREPARE TRANSACTION is not intended for use in applications or
       interactive sessions. Its purpose is to allow an	external transaction
       manager to perform atomic global	transactions across multiple databases
       or other	transactional resources. Unless	you're writing a transaction
       manager,	you probably shouldn't be using	PREPARE	TRANSACTION.

       This command must be used inside	a transaction block. Use 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
       prepared.

       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 Section 24.1.5,
	   "Preventing Transaction ID Wraparound Failures", 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 prepared transaction will normally be	committed or rolled
	   back	as soon	as an external transaction manager has verified	that
	   other databases 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';

COMPATIBILITY
       PREPARE TRANSACTION is a	PostgreSQL extension. It is intended for use
       by external transaction management systems, some	of which are covered
       by standards (such as X/Open XA), but the SQL side of those systems is
       not standardized.

SEE ALSO
       COMMIT PREPARED (COMMIT_PREPARED(7)), ROLLBACK PREPARED
       (ROLLBACK_PREPARED(7))

PostgreSQL 9.6.3		     2017		PREPARE	TRANSACTION(7)

NAME | SYNOPSIS | DESCRIPTION | PARAMETERS | NOTES | EXAMPLES | COMPATIBILITY | SEE ALSO

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