Cardinality Estimation¶
Accurate cardinality estimation is crucial for plan selection.
Statistics Collected¶
| Statistic | Purpose |
|---|---|
| Row count | Base cardinality |
| Distinct count | Join estimation |
| Histograms | Range selectivity |
| Null fraction | Null handling |
Selectivity Estimation¶
// Equality predicate
selectivity = 1 / distinct_count
// Range predicate
selectivity = (high - low) / (max - min)
// Join
output_rows = (rows_a * rows_b) / max(distinct_a, distinct_b)