What is index selectivity?

Selectivity describes how effectively a column or predicate narrows the number of matching rows. A highly selective predicate returns a relatively small fraction of the table.

Indexes on highly selective access paths are often useful for seeks, but selectivity is not the only design factor. Join patterns, ordering, covering needs, data distribution, update frequency, and query frequency also matter.

A low-selectivity column such as a BIT flag may still be useful as part of a filtered or composite index in the right workload.