The selectivity of an index is the result of evaluating the number of rows that can be selected in a search on every index value.A unique index has the maximum selectivity because it is impossible to select more than one row for each value of an index key if it is used.Keeping the selectivity of an index up to date is important for the optimizer’s choices in seeking the most optimal query plan.

Index statistics in Firebird are not automatically recalculated in response to large batches of inserts, updates or deletions.It may be beneficial to recalculate the selectivity of an index after such operations because the selectivity tends to become outdated.

It can be performed under concurrent load without risk of corruption.However, under concurrent load, the newly calculated statistics could become outdated as soon as SET STATISTICS finishes.

[fblangref50-ddl-idx-create], [fblangref50-ddl-idx-altridx]