|The PostgreSQL 9.0 Reference Manual - Volume 1B - SQL Command Reference
by The PostgreSQL Global Development Group
Paperback (6"x9"), 488 pages
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1.118 PREPARE TRANSACTION
PREPARE TRANSACTION -- prepare the current transaction for two-phase commit
PREPARE TRANSACTION transaction_id
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
Once prepared, a transaction can later be committed or rolled back
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,
TRANSACTION is not unlike a
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.)
PREPARE TRANSACTION command fails for any
reason, it becomes a
ROLLBACK: the current transaction
An arbitrary identifier that later identifies this transaction for
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.
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
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,
created any cursors
WITH HOLD, or executed
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
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
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
VACUUMto reclaim storage, and in extreme cases could cause the database to shut down to prevent transaction ID wraparound (see Volume 3: Preventing Transaction ID Wraparound Failures). 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_transactionsto zero. This will prevent accidental creation of prepared transactions that might then be forgotten and eventually cause problems.
Prepare the current transaction for two-phase commit, using
foobar as the transaction identifier:
PREPARE TRANSACTION 'foobar';
|ISBN 9781906966058||The PostgreSQL 9.0 Reference Manual - Volume 1B - SQL Command Reference||See the print edition|