Re: failed to build any 5-way joins

"Alessandro Depase" <alessandro.depase@libero.it> writes:
> select  *
> from info where parent_infoid is null=20
> and fieldtypeid in (select fieldtypeid from users_auth_groups uag1, field_a=
> uth fa where userid =3D 8 and fa.groupid =3D uag1.groupid)=20
> and main_infoid in (select ic.infoid from info_category ic, category_auth c=
> a, users_auth_groups uag2=20
>   where ic.categoryid =3D ca.categoryid and uag2.userid =3D 8 and uag2.grou=
> pid =3D ca.groupid and read_write =3D 'W')=20

Hmm.  It's right, there's no way to construct a sub-plan that joins just
that number of relations, because of the constraint that the IN
sub-selects have to be fully formed before we can do IN processing.

A brute force solution is to just remove the error cross-check in 
src/backend/optimizer/path/joinrels.c:
       if (result_rels == NIL)           elog(ERROR, "failed to build any %d-way joins", level);

I'll probably install some less-drastic fix for 7.4.1, but if you need a
solution right now, that will get you going.
        regards, tom lane