If you see “ORA-01555: snapshot too old” from Select what is been run on Physical standby, but same select runs on primary just fine, and all relevant init.ora parameters and undo tablespace are exactly identical between primary and standby,- its odd. But its easy to explain. First check if DB is below 22.214.171.124 DB it could be bug 10320455. If not – it most probably just undo retention problem. On Primary instance Increase undo_retention to value (in seconds) higher than time query runs, and it should fix the problem.