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In branches before commit 4be058fe9, we have to prevent flattening of subqueries with empty jointrees if the subqueries' output columns might need to be wrapped in PlaceHolderVars. That's because the empty jointree would result in empty phrels for the PlaceHolderVars, meaning we'd be unable to figure out where to evaluate them. However, we've failed to keep is_simple_subquery's check for this hazard in sync with what pull_up_simple_subquery actually does. The former is checking "lowest_outer_join != NULL", whereas the conditions pull_up_simple_subquery actually uses are if (lowest_nulling_outer_join != NULL) if (containing_appendrel != NULL) if (parse->groupingSets) So the outer-join test is overly conservative, while we missed out checking for appendrels and groupingSets. The appendrel omission is harmless, because in that case we also check is_safe_append_member which will also reject such subqueries. The groupingSets omission is a bug though, leading to assertion failures or planner errors such as "variable not found in subplan target lists". is_simple_subquery has access to none of the three variables used in the correct tests, but its callers do, so I chose to have them pass down a bool corresponding to the OR of these conditions. (The need for duplicative conditions would be a maintenance hazard in actively-developed code, but I'm not too concerned about it in branches that have only ~ 1 year to live.) Per bug #17614 from Wei Wei. Patch v10 and v11 only, since we have a better answer to this in v12 and later (indeed, the faulty check in is_simple_subquery is gone entirely). Discussion: https://postgr.es/m/17614-8ec20c85bdecaa2a@postgresql.org
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