|PostgreSQL Reference Manual - Volume 1 - SQL Language Reference|
by The PostgreSQL Global Development Group
Paperback (6"x9"), 716 pages
RRP £32.00 ($49.95)
Sales of this book support the PostgreSQL project! Get a printed copy>>>
12.89 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.
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
created any cursors
WITH HOLD. 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
From a performance standpoint, it is unwise to leave transactions in
the prepared state for a long time: this will for instance interfere with
the ability of
VACUUM to reclaim storage. Keep in mind also
that the transaction continues to hold whatever locks it held.
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 make any serious use of prepared transactions, you will probably
want to increase the value of
max_prepared_transactions, as the default setting is
quite small (to avoid wasting resources for those who don't use it).
It is recommendable to make it at least equal to
max_connections, so that every session can have
a prepared transaction pending.
Prepare the current transaction for two-phase commit, using
foobar as the transaction identifier:
PREPARE TRANSACTION 'foobar';
|ISBN 0954612027||PostgreSQL Reference Manual - Volume 1 - SQL Language Reference||See the print edition|