This signifies that a new transaction has been started. This indicates that a transaction was already in progress. The current transaction is not affected. Execution of multiple statements inside a transaction is also required for consistency when changing several related tables. If the transaction is committed, PostgreSQL will ensure either that all updates are done or else that none of them are done.
Notes Refer to LOCK for further information about locking tables inside a transaction.
You are advised to be careful about the transaction semantics when porting database applications.