PREPARE TRANSACTION
PREPARE TRANSACTION
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 PREPAREDorROLLBACK 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 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.