Measures of Query Cost(Cont.) Several algorithms can reduce disk IO by using extra buffer space Amount of real memory available to buffer depends on other concurrent queries and OS processes,known only during execution We often use worst case estimates,assuming only the minimum amount of memory needed for the operation is available Required data may be buffer resident already,avoiding disk I/O But hard to take into account for cost estimation Database System Concepts-6th Edition 12.9 @Silberschatz,Korth and SudarshanDatabase System Concepts - 6 12.9 ©Silberschatz, Korth and Sudarshan th Edition Measures of Query Cost (Cont.) Several algorithms can reduce disk IO by using extra buffer space Amount of real memory available to buffer depends on other concurrent queries and OS processes, known only during execution  We often use worst case estimates, assuming only the minimum amount of memory needed for the operation is available Required data may be buffer resident already, avoiding disk I/O But hard to take into account for cost estimation