Measures of Query Cost Cost is generally measured as total elapsed time for answering query Many factors contribute to time cost disk accesses,CPU,or even network communication Typically disk access is the predominant cost,and is also relatively easy to estimate.Measured by taking into account Number of seeks average-seek-cost Number of blocks read average-block-read-cost Number of blocks written average-block-write-cost Cost to write a block is greater than cost to read a block data is read back after being written to ensure that the write was successful Database System Concepts-6th Edition 12.7 ©Silberschat乜，Korth and SudarshanDatabase System Concepts - 6 12.7 ©Silberschatz, Korth and Sudarshan th Edition Measures of Query Cost Cost is generally measured as total elapsed time for answering query Many factors contribute to time cost  disk accesses, CPU, or even network communication Typically disk access is the predominant cost, and is also relatively easy to estimate. Measured by taking into account Number of seeks * average-seek-cost Number of blocks read * average-block-read-cost Number of blocks written * average-block-write-cost  Cost to write a block is greater than cost to read a block – data is read back after being written to ensure that the write was successful