Re: slow seqscan

> dba400=# explain analyze SELECT *, oid FROM annuncio400 WHERE rubric =
> 'DD' AND LOWER(testo) Like LOWER('cbr%') OFFSET 0 LIMIT 11;
> QUERY
> PLAN
> --------------------------------------------------------------------------------------------------------------------
>
> Limit (cost=0.00..3116.00 rows=11 width=546) (actual time=51.47..56.42
> rows=11 loops=1)
> -> Seq Scan on annuncio400 (cost=0.00..35490.60 rows=125 width=546)
> (actual time=51.47..56.40 rows=12 loops=1)
> Filter: ((rubric = 'DD'::bpchar) AND (lower((testo)::text) ~~
> 'cbr%'::text))
> Total runtime: 56.53 msec
> (4 rows)

What happens if you go:

CREATE INDEX annuncio400_rubric_testo_idx ON annuncio400(rubric,
LOWER(testo));

or even just:

CREATE INDEX annuncio400_rubric_testo_idx ON annuncio400(LOWER(testo));

> But the strangest thing ever is that if I change the filter with another
> one that represent a smaller amount of data it uses the index scan!!!

What's strange about that? The less data is going to be retrieved, the
more likely postgres is to use the index.

I suggest maybe increasing the amount of stats recorded for your rubrik
column:

ALTER TABLE annuncio400 ALTER rubrik SET STATISTICS 100;
ANALYZE annuncio400;

You could also try reducing the random_page_cost value in your
postgresql.conf a little, say to 3 (if it's currently 4). That will
make postgres more likely to use index scans over seq scans.

Chris