Updating sorted index prefix cardinalities as estimated values.

  Explanation:  The  RMU  Analyze  Cardinality  command  estimates
  index    prefix   cardinalities   when   actual   sorted   index
  cardinalities  are  updated.   It  then  updates  them  in   the
  RDB$INDEX_SEGMENTS   system   tables   so   that   index  prefix
  cardinalities will remain up-to-date.