PostgreSQL supports a two-phase commit (2PC)
   protocol that allows multiple distributed systems to work together
   in a transactional manner.  The commands are PREPARE
   TRANSACTION, COMMIT PREPARED and
   ROLLBACK PREPARED.  Two-phase transactions
   are intended for use by external transaction management systems.
   PostgreSQL follows the features and model
   proposed by the X/Open XA standard, but does not implement some less
   often used aspects.
  
   When the user executes PREPARE TRANSACTION, the
   only possible next commands are COMMIT PREPARED
   or ROLLBACK PREPARED. In general, this prepared
   state is intended to be of very short duration, but external
   availability issues might mean transactions stay in this state
   for an extended interval. Short-lived prepared
   transactions are stored only in shared memory and WAL.
   Transactions that span checkpoints are recorded in the
   pg_twophase directory.  Transactions
   that are currently prepared can be inspected using pg_prepared_xacts.