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PREPARE TRANSACTION()		 SQL Commands		 PREPARE TRANSACTION()

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 com‐
       mit.  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(5)]	or  ROLLBACK  PREPARED	[roll‐
       back_prepared(5)], 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  transac‐
       tion 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(5)] to start one.

       It  is not currently allowed to PREPARE a transaction that has executed
       any operations involving temporary tables or created any	 cursors  WITH
       HOLD.  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,	 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_pre‐
       pared_xacts system view.

       From a performance standpoint, it is unwise to  leave  transactions  in
       the  prepared  state  for a long time: this will for instance interfere
       with the ability of VACUUM to reclaim storage. 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 make any serious use of prepared transactions, you will probably
       want to increase the value of max_prepared_transactions, as the default
       setting is quite small (to avoid wasting resources for those who	 don't
       use  it).  It is recommendable to make it at least equal to max_connec‐
       tions, so that every session can have a prepared transaction pending.

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

       PREPARE TRANSACTION 'foobar';

SEE ALSO
       COMMIT  PREPARED [commit_prepared(5)], ROLLBACK PREPARED [rollback_pre‐
       pared(l)]

SQL - Language Statements	  2008-01-03		 PREPARE TRANSACTION()
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