Determining the cost of the query is the main job of the optimizer. The costs of columns used in joins come from statistics. The explain plan is not stored for a given query because the data may have changed between executions.
Row Estimates:
- May be estimated using random samples, statistics, or indexes
- Are assigned a confidence level - high, low, or none
- Affect timing estimates - more rows, more time needed
Timings:
- Used to determine the ‘lowest cost’ plan
- Total cost generated if all processing steps have assigned costs
- Not intended to predict wall-clock time, useful for comparisons
Miscellaneous Notes:
- Estimates too large to display show three asterisks (***)
- The accuracy of the time estimate depends upon the accuracy ...
