Re: Query with order by and limit is very slow - wrong index used

Lowering random_page_cost didn't help -- I've tried values 2.0 and 1.5.

Then I tried "order by id -1" hack Marcin Mańk proposed...

a9-dev=> create index foo on records(source_id, (id - 1));
CREATE INDEX
a9-dev=> explain analyze select * from records where source_id ='http://www.wbc.poznan.pl/dlibra/oai-pmh-repository.xml' order by (id -1) limit 200;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=0.00..379.42 rows=200 width=1124) (actual time=0.137..255.283 rows=200 loops=1)
-> Index Scan using foo on records (cost=0.00..1864617.14 rows=982887 width=1124) (actual time=0.137..255.237 rows=200 loops=1)
Index Cond: ((source_id)::text = 'http://www.wbc.poznan.pl/dlibra/oai-pmh-repository.xml'::text)
Total runtime: 255.347 ms
(4 rows)

Significant improvement :)

As we can see, it is possible to query records fast without changing table structure. Question is: can I do it without "hacks"?

Michal Nowak