PREPARE TRANSACTION() SQL Commands PREPARE TRANSACTION()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 com‐
mit. 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 transac‐
tion 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 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 pre‐
pared.
If the transaction modified any run-time parameters with SET, 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_pre‐
pared_xacts system view.
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. 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 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_connec‐
tions, so that every session can have a prepared transaction pending.
EXAMPLES
Prepare the current transaction for two-phase commit, using foobar as
the transaction identifier:
PREPARE TRANSACTION 'foobar';
SEE ALSO
COMMIT PREPARED [commit_prepared(5)], ROLLBACK PREPARED [rollback_pre‐
pared(l)]
SQL - Language Statements 2009-12-09 PREPARE TRANSACTION()
