Индекс — sorted auxiliary structure (B-tree/hash) с key + pointer к row, enabling binary search O(log n) vs scan O(n).
Разбор
- Cardinality matters: index on gender (2 values) rarely helps.
- Duplicate values: index finds first match, then scan leaf chain.
- Composite index: leftmost prefix rule for seek.
- Optimizer chooses seek vs scan based on stats.
Итог
Index = sorted keys + pointers; cardinality drives usefulness.
