Thread: subquery and table join, index not use for table

Hi, I have to following select:

set enable_seqscan = on;
set enable_indexscan =on;

select a.levelno,a.id from (select 1 as levelno,42 as id) a, menutable b
where  b.site_id='21' and a.id=b.id;

menutable:
id bigint,
site_id bigint

Indexes: menutable_pkey primary key btree (site_id, id),

The explain analyze shows:

  QUERY PLAN
----------------------------------------------------------------------------------------------------------------
  Nested Loop  (cost=0.00..13.50 rows=1 width=34) (actual
time=0.04..0.43 rows=1 loops=1)
    Join Filter: ("outer".id = "inner".id)
    ->  Subquery Scan a  (cost=0.00..0.01 rows=1 width=0) (actual
time=0.01..0.01 rows=1 loops=1)
          ->  Result  (cost=0.00..0.01 rows=1 width=0) (actual
time=0.00..0.00 rows=1 loops=1)
    ->  Seq Scan on menutable b  (cost=0.00..13.01 rows=38 width=22)
(actual time=0.02..0.38 rows=38 loops=1)
          Filter: (site_id = 21::bigint)
  Total runtime: 0.47 msec

setting set enable_seqscan = off;

QUERY PLAN

------------------------------------------------------------------------------------------------------------------------------------------
  Nested Loop  (cost=0.00..29.85 rows=1 width=34) (actual
time=0.07..0.18 rows=1 loops=1)
    Join Filter: ("outer".id = "inner".id)
    ->  Subquery Scan a  (cost=0.00..0.01 rows=1 width=0) (actual
time=0.01..0.01 rows=1 loops=1)
          ->  Result  (cost=0.00..0.01 rows=1 width=0) (actual
time=0.00..0.00 rows=1 loops=1)
    ->  Index Scan using menutable_pkey on menutable b
(cost=0.00..29.36 rows=38 width=22) (actual time=0.02..0.12 rows=38 loops=1)
          Index Cond: (site_id = 21::bigint)
  Total runtime: 0.22 msec

I do analyze, vacumm full analyze on table but nothing changed. The same
plan in case of join syntax.

version: PostgreSQL 7.3.3  and PostgreSQL 7.3.4

Any idea?
thx

C.

On Wed, 14 Jan 2004, CoL wrote:
[plan1]
>     ->  Seq Scan on menutable b  (cost=0.00..13.01 rows=38 width=22)
> (actual time=0.02..0.38 rows=38 loops=1)

[plan2]
>     ->  Index Scan using menutable_pkey on menutable b
> (cost=0.00..29.36 rows=38 width=22) (actual time=0.02..0.12 rows=38 loops=1)

It's estimating a cost of 13 for the sequence scan and 29 for the index
scan so it's choosing the sequence scan.

 The value of random_page_cost may be too high for your system and
especially so if the database is small and likely to fit in ram.  In
addition, if the table in question is small, you'll likely find that at
some point for a larger data set that the system switches over to an index
scan.