Re: [PERFORM] pushing order by + limit to union subqueries

On Sat, Feb 28, 2015 at 8:24 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> There would be cases where that would be a win, and there would be cases
> where it wouldn't be, so I'd not be in favor of making the transformation
> blindly. Unfortunately, given the current state of the planner that's
> all we could do really, because the subqueries are planned at arm's
> length and then we just mechanically combine them. Doing it "right" would
> entail fully planning each subquery twice, which would be very expensive.
>
Yes, after pulling up, subqueries are planned independently and we
glue them together finally.

> I have a longstanding desire to rewrite the upper levels of the planner to
> use path generation and comparison, which should make it more practical
> for the planner to compare alternative implementations of UNION and other
> top-level constructs. But I've been saying I would do that for several
> years now, so don't hold your breath :-(
>
GreenPlum utilizes Cascades optimizer framework (also used in SQL
Server and some others) to make the optimizer more modular and
extensible. In our context here, it allows thorough optimization
without pre-defined boundaries - no "subquery planning then glue
them". Is that something in your mind?

Regards,
Qingqing