inner join is much faster! is that right?

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=10000001000000

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=10000001000000

However this is not the case!

I have a many-to-many relation between 2 tables (tag and file). tag has
40000 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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