I was doing the explain thing when I discovered my mistake!
The think is that I over simplyfied my examples. instead of searching
for one ID, I was searching for several ids and I forgot to put a
parentesis arround the or like this.
I was doing this:
select *
from file a, file_tag b, tag c
where a.id_file=b.id_file and b.id_tag=c.id_tag and
a.id_file=10000001000000 or a.id_file=10000001000200
But I should have done this:
select *
from file a, file_tag b, tag c
where a.id_file=b.id_file and b.id_tag=c.id_tag and
(a.id_file=10000001000000 or a.id_file=10000001000200)
This changes everything. The performance is now acceptable!
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