g., frequently
executed queries), and can be used to answer multi-attribute queries directly.
The example PMap given here is handcrafted in order to illustrate terminology and
concepts. In practice, designing an efficient PMap is not a trivial task. We have
Indexing in Data Warehouses 8
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proposed algorithms that rely on heuristics to reduce the design space and create
efficient PMaps and are currently implementing them in an automated design tool
(Darira et al., 2006). One example of a design heuristic is how we determine the
details of range properties. We group all of the predicates over a single attribute in
a frequent query set together and create range intervals based on numeric constants
that appear in inequality comparisons. We create Boolean properties on equality
predicates and for attribute expressions that do not evaluate to numeric values. Enumerated
properties are created for attributes where the number of bits to represent
ranges over the attribute exceeds log2 of the number of values in the domain (since
this is the size of an enumerated property pstring.) Combinations of properties and
their ordering are evaluated based on reducing the number of excess tuples that
would be retrieved when evaluating the query set.
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