Re: seqscan for 100 out of 3M rows, index present

2013/6/26 Willy-Bas Loos <willybas(at)gmail(dot)com>

> postgres does a seqscan, even though there is an index present and it
> should be much more efficient to use it.
> I tried to synthetically reproduce it, but it won't make the same choice
> when i do.
> I can reproduce it with a simplified set of the data itself though.
>
> here's the query, and the analyzed plan:
> select count(*)
> from d2
> join g2 on g2.gid=d2.gid
> where g2.k=1942

1) Could you show the output of the following queries, please?
select relname,relpages,reltuples::numeric
from pg_class where oid in ('d2'::regclass, 'g2'::regclass);
select attrelid::regclass, attname,
CASE WHEN attstattarget<0 THEN
current_setting('default_statistics_target')::int4 ELSE attstattarget END
from pg_attribute
where attrelid in ('d2'::regclass, 'g2'::regclass) and attname='gid';

2) Will it help running the following?:
ALTER TABLE d2 ALTER gid SET STATISTICS 500;
VACUUM ANALYZE d2;
EXPLAIN (ANALYZE, BUFFERS) ...
SET enable_seqscan TO 'off';
EXPLAIN (ANALYZE, BUFFERS) ...

--
Victor Y. Yegorov