Re: Aggregate leads to superfluous projection from the scan

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

Ibrar Ahmed <ibrar.ahmad@gmail.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

     * For table scans, rather than using the relation targetlist (which is
     * only those Vars actually needed by the query), we prefer to generate a
     * tlist containing all Vars in order.  This will allow the executor to