Re: Cross Join Problem

Thanx alot... its solved my problem

On Mon, Aug 18, 2008 at 8:50 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> [ please keep the list cc'd for the archives' sake ]
>
> "Gauri Kanekar" <meetgaurikanekar(at)gmail(dot)com> writes:
> > On Mon, Aug 18, 2008 at 7:32 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> No PG release since 7.3 would have voluntarily planned that query that
> >> way. Maybe you were using join_collapse_limit = 1 to force the join
> >> order?
>
> > Yes, We have set join_collapse_limit set to 1.
>
> Ah, so really your question is why join_collapse_limit isn't working as
> you expect. That code changed quite a bit in 8.2, and the way it works
> now is that the critical decision occurs while deciding whether to fold
> the cross-join (a sub-problem of size 2) into the top-level join
> problem. Which is a decision that's going to be driven by
> from_collapse_limit not join_collapse_limit.
>
> So one way you could make it work is to reduce from_collapse_limit to
> less than 3, but I suspect you'd find that that has too many bad
> consequences for other queries. What's probably best is to write the
> problem query like this:
>
> FROM table1 a cross join ( table2 b cross join table3 c )
>
> which will cause join_collapse_limit to be the relevant number at both
> steps.
>
> regards, tom lane
>

--
Regards
Gauri