Re: Aggregate leads to superfluous projection from the scan

On Fri, Jul 8, 2022 at 12:30 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Ibrar Ahmed <ibrar(dot)ahmad(at)gmail(dot)com> writes:
> > I give a quick look and I think in case whenever data is extracted from
> the
> > heap it shows all the columns. Therefore when columns are extracted from
> > the index only it shows the indexed column only.
>
> This is operating as designed, and I don't think that the proposed
> patch is an improvement. The point of use_physical_tlist() is that
> returning all the columns is cheaper because it avoids a projection
> step. That's true for any case where we have to fetch the heap
> tuple, so IndexScan is included though IndexOnlyScan is not.
>
> Now, that's something that was true a decade or more ago.
> There's been considerable discussion recently about cases where
> it's not true anymore, for example with columnar storage or FDWs,
> and so we ought to invent a way to prevent createplan.c from
> doing it when it would be counterproductive. But just summarily
> turning it off is not an improvement.
>
> regards, tom lane
>
Hi,
In createplan.c, there is `change_plan_targetlist`

Plan *
change_plan_targetlist(Plan *subplan, List *tlist, bool tlist_parallel_safe)

But it doesn't have `Path` as parameter.
So I am not sure whether the check of non-returnable columns should be done
in change_plan_targetlist().

bq. for example with columnar storage or FDWs,

Yeah. The above is the case where I want to optimize.

Cheers