Re: Aggregate leads to superfluous projection from the scan

On Fri, Jul 8, 2022 at 12:48 PM Zhihong Yu <zyu(at)yugabyte(dot)com> wrote:

>
>
> 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
>
Hi, Tom:
I was looking at the following comment in createplan.c :

* 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
* optimize away projection of the table tuples, if possible.

Maybe you can give me some background on the above decision.

Thanks