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 or
       ROLLBACK PREPARED, 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 to start one.

       It is not currently allowed to PREPARE a transaction that has executed any operations
       involving temporary tables or the session's temporary namespace, created any cursors WITH
       HOLD, or executed LISTEN, UNLISTEN, or NOTIFY. 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 25.1.5). 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))