The following is from a database of several hundred million rows of real data that has been VACUUM ANALYZEd.
Why isn't the index being used for a query that seems tailor-made for it? The results (6,300 rows) take about ten minutes to retrieve with a sequential scan.
A copy of this database with "integer" in place of "smallint", a primary key in column order (date, time, type, subtype) and a secondary index in the required order (type, subtype, date, time) correctly uses the secondary index to return results in under a second.
Actually, the integer version is the first one I made, and the smallint is the copy, but that shouldn't matter.
Postgres is version "postgresql-server-7.3.4-3.rhl9" from Red Hat Linux 9.
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testdb2=# \d db
Table "public.db"
Column | Type | Modifiers
---------+------------------------+-----------
date | date | not null
time | time without time zone | not null
type | smallint | not null
subtype | smallint | not null
value | integer |
Indexes: db_pkey primary key btree ("type", subtype, date, "time")
testdb2=# set enable_seqscan to off;
SET
testdb2=# explain select * from db where type=90 and subtype=70 and date='7/1/2004';
QUERY PLAN
------------------------------------------------------------------------------
Seq Scan on db (cost=100000000.00..107455603.76 rows=178 width=20)
Filter: (("type" = 90) AND (subtype = 70) AND (date = '2004-07-01'::date))
(2 rows)