I have a large table with a column:
ids integer[] not null
most of these entries (over 95%) contain only one array element, some
can contain up to 10 array elements. seqscan is naturally slow. GIST
on int_array works nice, but GIST isn't exactly a speed daemon when it
comes to updating.
So I thought, why not create partial indexes?
CREATE INDEX one_element_array_index ON table ((ids[1])) WHERE icount(ids) <= 1;
CREATE INDEX many_element_array_index ON table USING GIST (ids) WHERE
icount(ids) > 1;
Now, if I select WHERE icount(ids) <= 1 AND ids[1] = 33 I get
lightning fast results.
If I select WHERE icount(ids) > 1 AND ids && '{33}' -- I get them even faster.
But when I phrase the query:
SELECT * FROM table WHERE (icount(ids) <= 1 AND ids[1] = 33) OR
(icount(ids) > 1 AND ids && '{33}');
Planner insists on using seqscan. Even with enable_seqscan = off;
Any hints, comments? :) [ I think thsese partial indexes take best of
two worlds, only if planner wanted to take advantage of it... :) ]
Regards,
Dawid