1. statistics not up to date, lost an index, parameter changes, table growed which is related to statistics
2. from memory of course, you get those statistics from shared pool
3. add a function to your predicate or add a number

where upper(X) = 'X'

where to_char(hiredate, 'YYYYMMDD') = '20030504'

where salary + 0 = '1000000'