Re: Outer joins and equivalence

On Mon, 2008-06-02 at 18:10 +0100, Simon Riggs wrote:
> On Tue, 2008-05-27 at 17:43 -0400, Tom Lane wrote:
> > Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> > > I have a complex query where making a small change to the SQL increases
> > > run-time by > 1000 times.
> >
> > > The first SQL statement is of the form
> >
> > > A JOIN B ON (a.id = b.id) LEFT JOIN C ON (a.id = c.id)
> >
> > > and the second is like this
> >
> > > A JOIN B ON (a.id = b.id) LEFT JOIN C ON (b.id = c.id)
> >
> > > the only difference is the substitution of a -> b
> >
> > Please provide an actual test case.
>
> Getting closer, but still not able to produce a moveable test case.

I've got a test case which shows something related and weird, though not
the exact case.

The queries shown here have significantly different costs, depending
upon whether we use tables a or b in the query. Since a and b are
equivalent this result isn't expected at all.

I suspect the plan variation in the original post is somehow cost
related and we are unlikely to discover the exact plan.

--
Simon Riggs www.2ndQuadrant.com
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