Measures of Query Cost(Cont.) Required data may be buffer resident already,avoiding disk l/O But hard to take into account for cost estimation 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 Worst case estimates assume that no data is initially in buffer and only the minimum amount of memory needed for the operation is available But more optimistic estimates are used in practice Database System Concepts-7th Edition 15.9 ©Silberscha乜，Korth and SudarshanDatabase System Concepts - 7 15.9 ©Silberschatz, Korth and Sudarshan th Edition Measures of Query Cost (Cont.) ▪ Required data may be buffer resident already, avoiding disk I/O • But hard to take into account for cost estimation ▪ 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 ▪ Worst case estimates assume that no data is initially in buffer and only the minimum amount of memory needed for the operation is available • But more optimistic estimates are used in practice