The planner needs to treat GroupingFunc like Aggref for many purposes,
in particular with respect to processing of the argument expressions,
which are not to be evaluated at runtime.  A few places hadn't gotten
that memo, notably including subselect.c's processing of outer-level
aggregates.  This resulted in assertion failures or wrong plans for
cases in which a GROUPING() construct references an outer aggregation
level.

Also fix missing special cases for GroupingFunc in cost_qual_eval
(resulting in wrong cost estimates for GROUPING(), although it's
not clear that that would affect plan shapes in practice) and in
ruleutils.c (resulting in excess parentheses in pretty-print mode).

Per bug #17088 from Yaoguang Chen.  Back-patch to all supported
branches.

Richard Guo, Tom Lane

Discussion: https://postgr.es/m/17088-e33882b387de7f5c@postgresql.org