Model
Selectivity under independence
Only true if the columns are independent. They usually are not.
- Selectivity
- the fraction of rows a predicate keeps
Worked examples, recomputed
Every number the mechanisms quote for this relationship is recomputed from it, so none of them disagrees with it.
- 1,000,000 × 0.02 × 0.001 = 20 rows estimated; the truth is 1,000
- Oslo and NO: 0.5% estimated, 5% true — ten times off
Where it appears
Each mechanism states it in its own names. The general symbol each name stands for is shown with it.
- Query Processing
Why can one question be answered by plans whose costs differ by a factor of ten thousand, and what does each operator force the engine to do?
- Query Planning & Optimization
How does a system choose a plan for a question when the facts that decide the best plan are unknown until the plan has run?