Ключников А.С. wrote:
And
select * from base
where id in (1,2) and datatime between '2005-05-15' and '2005-05-17';
10 seconds
select * from base
where id in (select id from device where id = 1 or id = 2) and
datatime between '2005-05-15' and '2005-05-17';
10 minits
Why?
Run EXPLAIN ANALYSE on both queries to see how the plan has changed.
My guess for why the plans are different is that in the first case your
query ends up as ...where (id=1 or id=2)...
In the second case, the planner doesn't know what it's going to get back
from the subquery until it's executed it, so can't tell it just needs to
scan base_1,base_2. Result: you'll scan all child tables of base.
I think the planner will occasionally evaluate constants before
planning, but I don't think it will ever execute a subquery and then
re-plan the outer query based on those results. Of course, someone might
pop up and tell me I'm wrong now...
--
Richard Huxton
Archonet Ltd
