On Fri, 2009-07-10 at 09:46 -0400, Bruce Momjian wrote:
> Simon Riggs wrote:
> > or a query like this
> >
> > Select '1', ...
> > ...
> > union
> > Select status, ...
> > ...
> > where status != '1';
> > ;
> >
> > then it is clear that we could automatically prove that the the distinct
> > step is redundant and so we could either hash or sort. This is the same
> > as replacing the UNION with UNION ALL.
>
> In the last example, how do you know that status != '1' produces unique
> output?
You don't. I was assuming that you could already prove that each
subquery was distinct in itself.
It's one for the TODO, that's all. I see it often, but I'm not planning
to work on the code for this myself.
--
Simon Riggs www.2ndQuadrant.com
PostgreSQL Training, Services and Support
