Matching index scans can use qualifiers that further restrict the result set. Remember that a WHERE clause that contains at least one optimizable predicate is optimizable. Nonoptimizable predicates can be useful in other ways.

Consider the following query: SELECT * FROM FLIGHTS WHERE orig_airport < 'BBB' AND orig_airport <> 'AKL'

The second predicate is not optimizable, but the first predicate is. The second predicate becomes a qualification for which evaluates the entries in the index as it traverses it.

• The following comparisons are valid qualifiers:
  • =
  • <
  • <=
  • >
  • >=
  • IS NULL
  • BETWEEN
  • LIKE
  • <>
  • IS NOT NULL
• The qualifier's reference to the column does not have to be a simple column reference; you can put the column in an expression.
• The qualifier's column does not have to be the first column in the index and does not have to be contiguous with the first column in the index.