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

Well, I think what happens here is that the new SJE code alters the
subquery (by removing ref_2) and that change is not propagated into the
parent level.  So in the parent level, when we look at the subquery's
original targetlist (which remains unchanged) against 'subroot' (which
has been changed accordingly), we'd have problem.

AFAICS there are two solutions being discussed:

1) We propagate the change to the subquery into the parent level by
ensuring that root->parse references the same Query structure during the
whole subquery_planner().

2) In the parent level, we look at the changed subquery instead of the
original rte->subquery.  IOW, we look at subroot->parse->targetList
instead of subquery->targetList.

Personally I prefer the second solution because it seems more natural to
look at 'subroot->parse' together with 'subroot' in estimate_num_groups
and it does not introduce new constraint to subquery_planner().