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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 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 exe-
       cuting  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 can-
       celed.

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 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 pre-
              vent  transaction  ID wraparound (see in the documentation). Keep in mind also that the transaction con-
              tinues 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';


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



SQL - Language Statements         2014-02-17            PREPARE TRANSACTION(7)