Re: ERROR: ORDER/GROUP BY expression not found in targetlist

Amit Kapila <amit(dot)kapila(at)enterprisedb(dot)com> writes:
> On Mon, Jun 13, 2016 at 7:51 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> I think the real question here is why the code removed by 04ae11f62
>> was wrong. It was unsafe to use apply_projection_to_path, certainly,
>> but using create_projection_path directly would have avoided the
>> stated problem. And it's very unclear that this new patch doesn't
>> bring back that bug in a different place.

> This new patch still doesn't seem to be right, but it won't bring back the
> original problem because apply_projection_to_path will be only done if
> grouped_rel is parallel_safe which means it doesn't have any
> parallel-unsafe or parallel-restricted clause in quals or target list.

The problem cited in 04ae11f62's commit message is that
apply_projection_to_path would overwrite the paths' pathtargets in-place,
causing problems if they'd been used for other purposes elsewhere. I do
not share your confidence that using apply_projection_to_path within
create_grouping_paths is free of such a hazard.

regards, tom lane