Re: wCTE behaviour

On Fri, Nov 12, 2010 at 10:50:52AM -0500, Robert Haas wrote:
> On Fri, Nov 12, 2010 at 10:25 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > Yeb Havinga <yebhavinga(at)gmail(dot)com> writes:
> >> On 2010-11-11 17:50, Marko Tiikkaja wrote:
> >>> Just to be clear, the main point is whether they see the data
> >>> modifications or not.  The simplest case to point out this behaviour is:
> >>>
> >>> WITH t AS (DELETE FROM foo)
> >>> SELECT * FROM foo;
> >>>
> >>> And the big question is: what state of "foo" should the SELECT
> >>> statement see?
> >
> >> Since t is not referenced in the query, foo should not be deleted at
> >> all,
> >
> > Yeah, that's another interesting question: should we somehow force
> > unreferenced CTEs to be evaluated anyhow?  Now that I think about it,
> > there was also some concern about the possibility of the outer query
> > not reading the CTE all the way to the end, ie
> >
> >        WITH t AS (DELETE FROM foo RETURNING *)
> >        SELECT * FROM t LIMIT 1;
> >
> > How many rows does this delete?  I think we concluded that we should
> > force the DELETE to be run to conclusion even if the outer query didn't
> > read it all.  From an implementation standpoint that makes it more
> > attractive to do the DELETE first and stick its results in a tuplestore
> > --- but I still think we should view that as an implementation detail,
> > not as part of the specification.
>
> Yeah, I think we have to force any DML statements in CTEs to run to
> completion, whether we need the results or not, and even if they are
> unreferenced. Otherwise it's going to be really confusing, I fear.

Yes, and as we add more things--COPY is the first but probably not the
last--to CTEs, this "no action-at-a-distance" behavior will become
even more important.

Cheers,
David.
--
David Fetter <david(at)fetter(dot)org> http://fetter.org/
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