| PREPARE_TRANSACTION(7) - phpMan
PREPARE TRANSACTION(7) PostgreSQL 12.3 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
(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))
PostgreSQL 12.3 2020 PREPARE TRANSACTION(7)
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