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Estimating Query Cost

In the previous section, we understood about Query processing steps and evaluation plan. Though a system can create multiple plans for a query, the chosen method should be the best of all. It can be done by comparing each possible plan in terms of their estimated cost. For calculating the net estimated cost of any plan, the cost of each operation within a plan should be determined and combined to get the net estimated cost of the query evaluation plan.

The cost estimation of a query evaluation plan is calculated in terms of various resources that include:

  • Number of disk accesses
  • Execution time taken by the CPU to execute a query
  • Communication costs in distributed or parallel database systems.

To estimate the cost of a query evaluation plan, we use the number of blocks transferred from the disk, and the number of disks seeks. Suppose the disk has an average block access time of ts seconds and takes an average of tT seconds to transfer x data blocks. The block access time is the sum of disk seeks time and rotational latency. It performs S seeks than the time taken will be b*tT + S*tS seconds. If tT=0.1 ms, tS =4 ms, the block size is 4 KB, and its transfer rate is 40 MB per second. With this, we can easily calculate the estimated cost of the given query evaluation plan.

Generally, for estimating the cost, we consider the worst case that could happen. The users assume that initially, the data is read from the disk only. But there must be a chance that the information is already present in the main memory. However, the users usually ignore this effect, and due to this, the actual cost of execution comes out less than the estimated value.

The response time, i.e., the time required to execute the plan, could be used for estimating the cost of the query evaluation plan. But due to the following reasons, it becomes difficult to calculate the response time without actually executing the query evaluation plan:

  • When the query begins its execution, the response time becomes dependent on the contents stored in the buffer. But this information is difficult to retrieve when the query is in optimized mode, or it is not available also.
  • When a system with multiple disks is present, the response time depends on an interrogation that in "what way accesses are distributed among the disks?". It is difficult to estimate without having detailed knowledge of the data layout present over the disk.
  • Consequently, instead of minimizing the response time for any query evaluation plan, the optimizers finds it better to reduce the total resource consumption of the query plan. Thus to estimate the cost of a query evaluation plan, it is good to minimize the resources used for accessing the disk or use of the extra resources.

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