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
   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.
  
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.
     
   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.
  
    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.
   Prepare the current transaction for two-phase commit, using
   foobar as the transaction identifier:
PREPARE TRANSACTION 'foobar';
   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.