> From: Tom Lane [mailto:tgl@sss.pgh.pa.us]
> Alexander Korotkov <aekorotkov@gmail.com> writes:
> > On Mon, Dec 3, 2012 at 8:31 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >> But having said that, I'm wondering (without having read the patch)
> >> why you need anything more than the existing "resjunk" field.
>
> > Actually, I don't know all the cases when "resjunk" flag is set. Is it
> > reliable to decide target to be used only for "ORDER BY" if it's "resjunk"
> > and neither system or used in grouping? If it's so or there are some other
> > cases which are easy to determine then I'll remove "resorderbyonly" flag.
>
> resjunk means that the target is not supposed to be output by the query.
> Since it's there at all, it's presumably referenced by ORDER BY or GROUP
> BY or DISTINCT ON, but the meaning of the flag doesn't depend on that.
>
> What you would need to do is verify that the target is resjunk and not
> used in any clause besides ORDER BY. I have not read your patch, but
> I rather imagine that what you've got now is that the parser checks this
> and sets the new flag for consumption far downstream. Why not just make
> the same check in the planner?
I've created a patch using this approach. Please find attached the patch.
> A more invasive, but possibly cleaner in the long run, approach is to
> strip all resjunk targets from the query's tlist at the start of
> planning and only put them back if needed.
>
> BTW, when I looked at this a couple years ago, it seemed like the major
> problem was that the planner assumes that all plans for the query should
> emit the same tlist, and thus that tlist eval cost isn't a
> distinguishing factor. Breaking that assumption seemed to require
> rather significant refactoring. I never found the time to try to
> actually do it.
Such an approach would improve code readability, but I'm not sure it's worth the
work for this optimization, though I think I'm missing something.
Thanks,
Best regards,
Etsuro Fujita