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.
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.
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 prepared.
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_prepared_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_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';