On Jul 17, 2012, at 5:32 PM, Simon Riggs wrote: > CREATE INDEX ON foo (a, b, c, d); > > allows > > SELECT c, d FROM foo WHERE a = ? AND b = ? > > to use an index only scan. > > The phrase "unindexed" seems misleading since the data is clearly in > the index from the description on the URL you gave. And since the > index is non-unique, I don't see any gap between Postgres and > SQLliite4.
Yeah, but that index is unnecessarily big if one will never use c or d in the search. The nice thing about covering indexes as described for SQLite 4 and implemented in MSSQL is that you can specify additional columns that just come along for the ride, but are not part of the indexed data: CREATE INDEX cover1 ON table1(a,b) COVERING(c,d); Yes, you can do that by also indexing c and d as of 9.2, but it might be nice to be able to include them in the index as additional row data without actually indexing them. Best, David -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers