26 Automated Physical Database Design and Tuning
intersected list of RIDs can be much smaller than either of the original lists,
and thus much fewer RID lookups are needed, and (3) if the predicates use
equalities, specialized intersection algorithms can be used that return RIDs
in order of the clustered index, which accelerates subsequent lookups.
In general, we can intersect several indexes to answer complex single-table
predicates, although at some point the benefit from additional index intersec-
tions is outweighed by its cost. Additionally, there are analogous strategies to
handle disjunctive predicates using RID unions, and negation predicates using
RID differences or two seek operations (e.g., employees who are not engineers).
Considering these ...