[SQL] inner join is much faster! is that right?

[James Im]

I really thought that Postgresql would rewrite a query from

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=100100

to something like:

select *
from (file a inner join file_tag b on (a.id_file=b.id_file)) inner join
tag c on (b.id_tag=c.id_tag)
where a.id_file=100100


However this is not the case!

I have a many-to-many relation between 2 tables (tag and file). tag has
4 rows and file has 5 millions rows. file_tag has 50 millions rows.

In this setup, the first select takes a couple of minutes to complete
while the second query takes only a second to complete.

Is this a normal expected behavior? Shouldn't Postgresql be able to
rewrite the query correctly?

_
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Re: [SQL] inner join is much faster! is that right?

[James Im]

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=100100 or a.id_file=1001000200

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=100100 or a.id_file=1001000200)

This changes everything. The performance is now acceptable!

_
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