PREPARE TRANSACTION(7) PostgreSQL 15.7 Documentation 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 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))
PostgreSQL 15.7 2024 PREPARE TRANSACTION(7)
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