Measures of Query Cost (Cont.) For simplicity we just use the number of block transfers from disk and the number of seeks as the cost measures f-time to transfer one block ts-time for one seek Cost for b block transfers plus S seeks b *t+S *ts We ignore CPU costs for simplicity Real systems do take CPU cost into account We do not include cost to writing output to disk in our cost formulae 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 l/O But hard to take into account for cost estimation Database System Concepts-5th Edition,Aug 27,2005. 13.8 ©Silberschat乜，Korth and SudarshanDatabase System Concepts - 5 13.8 ©Silberschatz, Korth and Sudarshan th Edition, Aug 27, 2005. Measures of Query Cost (Cont.) For simplicity we just use the number of block transfers from disk and the number of seeks as the cost measures tT – time to transfer one block tS – time for one seek Cost for b block transfers plus S seeks b * tT + S * tS We ignore CPU costs for simplicity Real systems do take CPU cost into account We do not include cost to writing output to disk in our cost formulae 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