BUG #6498: with recursive / union all

The following bug has been logged on the website:

Bug reference:      6498
Logged by:          kanishka
Email address:      nish2575@gmail.com
PostgreSQL version: 9.0.6
Operating system:   Linux 2.6.41.4-1.fc15.i686 / fedora 15
Description:=20=20=20=20=20=20=20=20

here is an example of what i believe the system is not obeying the
expectation of a union all instead of a union, in a recursive with query
that uses a join. the documentation page on with queries also has an example
with union all over an acyclic graph data set that i believe should loop
infinitely according the descriptoin of with query processing.

this doesn't loop infinitely:

 insert into acyc values ('a','b') ,('b','c');

 with recursive paths as (
select frm, too, 1 as lvl from acyc
union all
select fnd.frm, a.too, lvl + 1 from acyc a join paths fnd on fnd.too =3D
a.frm ) select * from paths;


while this does loop infinitely:

 with recursive p as (select 5 union all select * from p ) select * from p;


my guess is the performing of the join is causing duplicates inadvertantly.