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
This command must be used inside a transaction block. Use BEGIN
[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
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 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.
EXAMPLES
Prepare the current transaction for two-phase commit, using foobar as
the transaction identifier:
PREPARE TRANSACTION ’foobar’;
SEE ALSO
COMMIT PREPARED [commit_prepared(7)], ROLLBACK PREPARED
[rollback_prepared(7)]