Horton

Re: Redundant Result node

Richard Guo <guofenglinux@gmail.com>

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From: Richard Guo <guofenglinux@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2024-08-23T03:48:37Z
Lists: pgsql-hackers

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  1. Avoid unnecessary post-sort projection

On Fri, Aug 23, 2024 at 11:19 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Richard Guo <guofenglinux@gmail.com> writes:
> > ... we'll always make a separate ProjectionPath on top of the SortPath
> > in create_ordered_paths.  It’s only when we create the plan node for
> > the projection step in createplan.c that we realize a separate Result
> > is unnecessary.  This is not efficient.
>
> I'm not sure you're considering "efficiency" in the right light.
> In my mind, any time we can postpone work from path-creation time
> to plan-creation time, we're probably winning because we create
> many more paths than plans.  Perhaps that's wrong in this case,
> but it's not anywhere near as obvious as you suggest.

I agree that it’s always desirable to postpone work from path-creation
time to plan-creation time.  In this case, however, it’s a little
different.  The projection step could actually be avoided from the
start if we perform the correct check in create_ordered_paths.

Thanks
Richard