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       PREPARE_TRANSACTION - prepare the current transaction for two-phase


       PREPARE TRANSACTION transaction_id


       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

       Once prepared, a transaction can later be committed or rolled back with
       (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.


           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.


       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

       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.

           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 23.1.5,
           “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.


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

           PREPARE TRANSACTION 'foobar';


       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.



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