Re: BUG #18170: Unexpected error: no relation entry for relid 3

On Fri, Oct 27, 2023 at 10:41 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:

> Richard Guo <guofenglinux@gmail.com> writes:
> > Alternatively, can we look at subroot->parse->targetList instead of
> > subquery->targetList where we call estimate_num_groups on the output of
> > the subquery?
>
> Please read the comment just above that.


Hmm, I understand that we cannot look at subroot->processed_tlist here
because of the "varno 0" hack explained in the comment.  But what I
proposed is to look at subroot->parse->targetList, which should be fine
because the tlist generated by generate_append_tlist would only be
returned into processed_tlist not the original parse->targetList.  For
instance, the query below still works if we look at
subroot->parse->targetList, even though the subquery is itself a setop.

(SELECT 1,2,3 UNION SELECT 4,5,6 ORDER BY 1,2) INTERSECT SELECT 4,5,6;
 ?column? | ?column? | ?column?
----------+----------+----------
        4 |        5 |        6
(1 row)

Thanks
Richard