Re: Predicate migration on complex self joins

On Mon, 2009-07-13 at 13:33 -0400, Tom Lane wrote:
> Simon Riggs <simon@2ndQuadrant.com> writes:
> > In some cases, we have SQL being submitted that has superfluous
> > self-joins. An example would be
> 
> > select count(*) 
> > from foo1 a, foo1 b 
> > where a.c1 = b.c1 /* PK join */
> > and a.c2 = 5 
> > and b.c2 = 10;
> 
> > You may well ask who would be stupid enough to write SQL like that. The
> > answer is of course that it is automatically generated by an ORM.
> 
> Seems like the right answer is "fix the damn ORM".  It's hard to believe
> this sort of case comes up often enough to justify the cycles that would
> be expended (on *every* join query) to try to recognize it.

Yeh, damn ORMs seem to spring up faster than vines.

Not just because of this but I wonder if we might benefit from an
optimizer setting specifically aimed at the foolishnesses of
automatically generated SQL.

-- Simon Riggs           www.2ndQuadrant.comPostgreSQL Training, Services and Support