Measure of Query Cost

Measure of Query Cost

AU: Dec.-19, Marks 3

• There are multiple possible evaluation plans for a query, and it is important to be able to compare the alternatives in terms of their estimated cost and choose the best plan.

• There are many factors that contribute to query cost are -

  • Disk access

  • CPU time to execute the query

  • Cost of communication in distributed and parallel database system.

• The cost of access data from disk is an important cost. Normally disk access is relatively slow as compared to in-memory operations. Moreover, the CPU speed is much faster than the disk speed. Hence the time spent in disk is relatively dominant factor in query execution.

• Computing CPU access time is comparatively harder, hence we will not consider the CPU time to execute the query.

• Similarly the cost of communication does not matter for simple large databases ero present in the centralized database system.

• Typically disk access is the predominant cost and is also relatively easy to estimate, 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 because data is read back after being written to ensure that the write was successful.

• We use number of block transfers from disk and number of disk seeks to estimate the Query cost.

• Let,

  • b be the number to blocks

  • S be the number of Seeks

  • t_T is average time required to transfer a block of data, in seconds

  • t_S is average block access time in seconds.