Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly.
COMMIT commits the current transaction. All changes made by the transaction become visible to others and are guaranteed to be durable if a crash occurs.
Use ROLLBACK to abort a transaction.
Issuing COMMIT when not inside a transaction does no harm, but it will provoke a warning message.
Note: The following description applies only to Postgres-XC
When more than one node are involved in a transaction, commit may be assosicated with inplicit 2PC and PREPARE command will be issued internaly. In this case, application may have additional restriction related to 2PC restrictions.