On Mon, Jul 14, 2014 at 3:00 AM, Tom Lane <t...@sss.pgh.pa.us> wrote:

> David Rowley <dgrowle...@gmail.com> writes:
> > I had another look at this and it appears you were right the first time,
> we
> > need to ensure there's no NULLs on both sides of the join condition.
> Ugh.  I'm back to being discouraged about the usefulness of the
> optimization.
Are you worried about the planning overhead of the not null checks, or is
it more that you think there's a much smaller chance of a real world
situation that the optimisation will succeed? At least the planning
overhead is limited to query's that have NOT IN clauses.

I'm still quite positive about the patch. I think that it would just be a
matter of modifying query_outputs_are_not_nullable() giving it a nice new
name and changing the parameter list to accept not only a Query, but also a
List of Expr. Likely this would be quite a nice reusable function that
likely could be used in a handful of other places in the planner to
optimise various other cases.

When I first decided to work on this I was more interested in getting some
planner knowledge about NOT NULL constraints than I was interested in
speeding up NOT IN, but it seemed like a perfect target or even "excuse" to
draft up some code that checks if an expr can never be NULL.

Since the patch has not been marked as rejected I was thinking that I'd
take a bash at fixing it up, but if you think this is a waste of time,
please let me know.


David Rowley

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