Subject index and statistics
Author scharloi

I do the following :

- Create tables with primary and foreign keys and index.
- Insert all data

Next I execute a select statement (join 5 tables on foreign key). The
spent time is about 0.10 s.

Next do a "SET STATISTICS" on all index.

Execute the same statement and I time the response to 0.64 s.

Then if i deactivate index on varchar column, i time the response to
0.11 ms.

I read in the "Firebird Data Definition" book :
<<For tables where the number of duplicate values in indexed columns
radically increases or decreases, periodically recomputing index
selectivity can improve performance. SET STATISTICS recomputes the
selectivity of an index.>>

My question is : Why the performance of a query can decrease after
recomputing index selectivity ?