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Predicate transformations
WHERE clauses with predicates joined
by OR are usually not optimizable. WHERE clauses with predicates joined by
AND are optimizable if at least one of the predicates
is optimizable. For example:
SELECT * FROM Flights
WHERE flight_id = 'AA1111'
AND segment_number <> 2
In this example, the first predicate is optimizable; the second predicate
is not. Therefore, the statement is optimizable.
Note: In a few
cases, a WHERE clause with predicates joined by OR can be transformed into
an optimizable statement. See OR transformations.
Derby can transform some predicates internally so that at least one
of the predicates is optimizable and thus the statement is optimizable. This
section describes the predicate transformations that Derby performs
to make predicates optimizable.
A predicate that uses the following comparison operators can sometimes
be transformed internally into optimizable predicates.
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