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 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 24.1.5). 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))