Index Selectivity
A measure of how effectively an index key distinguishes rows, often expressed through the fraction or distribution of matching values.
Database Context
Index selectivity describes how narrowly an indexed value or predicate identifies rows. Highly selective predicates can make targeted index access attractive, while low selectivity can make a sequential or full scan cheaper once random I/O and row lookups are considered.
Estimation Boundary
The number of distinct values is only a rough proxy. Skew, nulls, clustering, covering behavior, predicate shape, and correlations with other columns all affect the optimizer's real cost decision.
Related Database Concepts
- Execution Plan
- Cardinality Estimation
- B-tree
- Query Optimizer