Measures of Query Cost Many factors contribute to time cost disk access,CPU,and network communication Cost can be measured based on response time,i.e.total elapsed time for answering query,or total resource consumption We use total resource consumption as cost metric Response time harder to estimate,and minimizing resource consumption is a good idea in a shared database We ignore CPU costs for simplicity Real systems do take CPU cost into account Network costs must be considered for parallel systems We describe how estimate the cost of each operation We do not include cost to writing output to disk Database System Concepts-7th Edition 15.7 ©Silberscha乜，Korth and SudarshanDatabase System Concepts - 7 15.7 ©Silberschatz, Korth and Sudarshan th Edition Measures of Query Cost ▪ Many factors contribute to time cost • disk access, CPU, and network communication ▪ Cost can be measured based on • response time, i.e. total elapsed time for answering query, or • total resource consumption ▪ We use total resource consumption as cost metric • Response time harder to estimate, and minimizing resource consumption is a good idea in a shared database ▪ We ignore CPU costs for simplicity • Real systems do take CPU cost into account • Network costs must be considered for parallel systems ▪ We describe how estimate the cost of each operation • We do not include cost to writing output to disk