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:
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 =
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.
nish2575@gmail.com writes:
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 =
a.frm ) select * from paths;
Why would it? You don't have any loops in the data. In particular that
means there will be no join matches after a certain number of levels.
regards, tom lane
but {5} union {5} = {5, 5}
so ('a', 'c', 2) union all ('a','c', 2) = {('a', 'c', 2), ('a', 'c', 2)}
the second ('a', 'c', 2) should result from joining the latest working
table in paths which includes {('a', 'b', 1), ('b', 'c', 1), ('a',
'c', 2)} with {('a','b'), ('b', 'c')} specifically ('a', 'b', 1) with
('a', 'b')
maybe i'm stupid or not understanding some assumption, but i interpret
not removing duplicates within and between tables as stated in the
docs for the union all case, as not removing the a,c,2 above.
-kanishka
Show quoted text
On 2/29/12, Tom Lane <tgl@sss.pgh.pa.us> wrote:
nish2575@gmail.com writes:
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 =
a.frm ) select * from paths;Why would it? You don't have any loops in the data. In particular that
means there will be no join matches after a certain number of levels.regards, tom lane