postgresql - pk versus unique index

Question

In a PostgreSQL 9 database there is a table that contains a serial field X which is a PK (oid enabled), and other fields.

Using postgres's pgadmin with that table - a query takes 30 seconds.

If I add a unique index on the same field X - same query in pgadmin takes 3 seconds.

PKs are implicitly indexes: http://www.postgresql.org/docs/current/static/indexes-unique.html

So why does the explicit index make a difference?

Is this a pgadmin issue?

Do I need an explicit index for the PK field or not?

Answer 1

No difference here (pg 9.1.2), I think it is an artifact (schema, case-insignificance?)

DROP SCHEMA tmp CASCADE;
CREATE SCHEMA tmp ;
SET search_path=tmp;

CREATE TABLE lutser
        ( id INTEGER NOT NULL PRIMARY KEY
        , val INTEGER NOT NULL
        );
INSERT INTO lutser(id,val)
SELECT g, g %31
FROM generate_series(1,100000) g
        ;

DELETE FROM lutser WHERE random() < .5;

VACUUM ANALYZE lutser;

EXPLAIN ANALYZE
SELECT COUNT(*) FROM lutser
WHERE id >= 1000 AND id < 2000
        ;

CREATE INDEX lutser_id ON lutser(id);

VACUUM ANALYZE lutser;

EXPLAIN ANALYZE
SELECT COUNT(*) FROM lutser
WHERE id >= 1000 AND id < 2000
        ;

Results:

NOTICE:  drop cascades to table tmp.lutser
DROP SCHEMA
CREATE SCHEMA
SET
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "lutser_pkey" for table "lutser"
CREATE TABLE
INSERT 0 100000
DELETE 50051
VACUUM
                                                          QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=20.28..20.29 rows=1 width=0) (actual time=0.294..0.295 rows=1 loops=1)
   ->  Index Scan using lutser_pkey on lutser  (cost=0.00..19.03 rows=499 width=0) (actual time=0.015..0.216 rows=487 loops=1)
         Index Cond: ((id >= 1000) AND (id < 2000))
 Total runtime: 0.343 ms
(4 rows)

CREATE INDEX
VACUUM
                                                         QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=19.03..19.04 rows=1 width=0) (actual time=0.232..0.232 rows=1 loops=1)
   ->  Index Scan using lutser_id on lutser  (cost=0.00..17.79 rows=497 width=0) (actual time=0.033..0.185 rows=487 loops=1)
         Index Cond: ((id >= 1000) AND (id < 2000))
 Total runtime: 0.266 ms
(4 rows)