Statistics-based versus hard-wired selectivity

Derby determines the selectivity for a WHERE clause in one of two ways.

• Selectivity from cardinality statistics
  Cardinality statistics are computed by the Derby system and stored in the system tables.
• Selectivity from hard-wired assumptions
  In all other cases, Derby uses a fixed number that attempts to describe the percentage of rows that will probably be returned; it might not correspond to the actual selectivity of the operation in every case. It is an assumption hard-wired into the Derby system.