Chapter on indices intended for the User's Guide.

Currently not included in the UG, and this now only has a discussion of
 partial indices by Paul Aoki culled from the mailing lists.
But, didn't want to loose it...

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+
+Indices
+
+
+
+
+Partial Indices
+
+
+
+Author
+
+This is from a reply to a question on the e-mail list
+by Paul M. Aoki
+on 1998-08-11.
+
+
+
+A partial index
+is an index built over a subset of a table; the subset is defined by
+a predicate.  Postgres
+ supported partial indices with arbitrary
+predicates.  I believe IBM's db2 for as/400 supports partial indices
+using single-clause predicates.
+
+
+The main motivation for partial indices is this:
+if all of the queries you ask that can
+profitably use an index fall into a certain range, why build an index
+over the whole table and suffer the associated space/time costs?
+
+(There are other reasons too; see 
+ for details.)
+
+
+The machinery to build, update and query partial indices isn't too
+bad.  The hairy parts are index selection (which indices do I build?)
+and query optimization (which indices do I use?); i.e., the parts
+that involve deciding what predicate(s) match the workload/query in
+some useful way.  For those who are into database theory, the problems
+are basically analogous to the corresponding materialized view
+problems, albeit with different cost parameters and formulae.  These
+are, in the general case, hard problems for the standard ordinal 
+SQL
+types; they're super-hard problems with black-box extension types,
+because the selectivity estimation technology is so crude.
+
+
+Check ,
+,
+and
+
+for more information.