Re: [HACKERS] Common Table Expressions (WITH RECURSIVE) patch

> On Tue, 2008-09-09 at 13:45 +0900, Tatsuo Ishii wrote:
> > Thanks for the review.
> >
> > > The standard specifies that non-recursive WITH should be evaluated
> > > once.
> >
> > What shall we do? I don't think there's a easy way to fix this. Maybe
> > we should not allow WITH clause without RECURISVE?
>
> My interpretation of 7.13: General Rules: 2.b is that it should be
> single evaluation, even if RECURSIVE is present.
>
> The previous discussion was here:
>
> http://archives.postgresql.org/pgsql-hackers/2008-07/msg01292.php
>
> The important arguments in the thread seemed to be:
>
> 1. People will generally expect single evaluation, so might be
> disappointed if they can't use this feature for that purpose.
>
> 2. It's a spec violation in the case of volatile functions.
>
> 3. "I think this is a "must fix" because of the point about volatile
> functions --- changing it later will result in user-visible semantics
> changes, so we have to get it right the first time."
>
> I don't entirely agree with #3. It is user-visible, but only in the
> sense that someone is depending on undocumented multiple-evaluation
> behavior.
>
> Tom Lane said that multiple evaluation is grounds for rejection:
> http://archives.postgresql.org/pgsql-hackers/2008-07/msg01318.php
>
> Is there hope of correcting this before November?

According to Tom, to implement "single evaluation" we need to make big
infrastructure enhancement which is likely slip the schedule for 8.4
release which Tom does not want.

So as long as Tom and other people think that is a "must fix", there
seems no hope probably.

Anyway I will continue to work on existing patches...
--
Tatsuo Ishii
SRA OSS, Inc. Japan

> > I will try to fix this. However detecting the query being not a
> > non-linear one is not so easy.
>
> If we don't allow mutual recursion, the only kind of non-linear
> recursion that might exist would be multiple references to the same
> recursive query name in a recursive query, is that correct?
>
> > > * DISTINCT should supress duplicates:
> > >
> > > with recursive foo(i) as
> > > (select distinct * from (values(1),(2)) t
> > > union all
> > > select distinct i+1 from foo where i < 10)
> > > select * from foo;
> > >
> > > This outputs a lot of duplicates, but they should be supressed
> > > according to the standard. This query is essentially the same as
> > > supporting UNION for recursive queries, so we should either fix both for
> > > 8.4 or block both for consistency.
> >
> > I'm not sure if it's possible to fix this. Will look into.
> >
>
> Can't we just reject queries with top-level DISTINCT, similar to how
> UNION is rejected?
>
> > > * outer joins on a recursive reference should be blocked:
> > >
> > > with recursive foo(i) as
> > > (values(1)
> > > union all
> > > select i+1 from foo left join (values(1)) t on (i=column1))
> > > select * from foo;
> > >
> > > Causes an infinite loop, but the standard says using an outer join
> > > in this situation should be prohibited. This should be fixed for 8.4.
> >
> > Not an issue, I think.
>
> Agreed, Andrew Gierth corrected me here.
>
> Regards,
> Jeff Davis
>
>
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