The costs are calculated for every possible strategy.
This is necessary because
- there may be several strategies, of which the best has to be determined
- there are cases in which the search performed by means of an index is more costly than the sequential search.
The costs refer to the estimated number of I/O processes which will have to be performed as a consequence of the selected strategy.
These costs are also output as a result of the EXPLAIN statement (see chapter The EXPLAIN Statement).