While working on the expansion of virtual generated columns, Dean encountered $subject in [1], which can be reproduced using the query below.
create table t (a int); insert into t values (1); # select a, b from (select a, a as b from t) ss group by grouping sets(a, b) having b = 1; a | b ---+--- 1 | (1 row) Note that the having clause filters out the wrong row. In 90947674f, we fixed a similar issue by wrapping subquery outputs that are non-var expressions in PlaceHolderVars. This prevents const-simplification from merging them into the surrounding expression after subquery pullup and resulting in something that won't match the grouping set expression in setrefs.c. It seems that we still have loose ends with that fix. This issue illustrates that if the subquery's target list contains two or more identical Var expressions, we can also fail to match the Var expression to the expected grouping set expression. This is not related to const-simplification, but rather to how we match expressions to lower target items in setrefs.c. For sort/group expressions, we use ressortgroupref matching, which works well. For other expressions, we primarily rely on comparing the expressions to determine if they are the same. This is why, in the query above, the Var 'b' in the HAVING clause matches the first target entry of the subquery, rather than the expected second one. You might wonder why this issue wasn't fixed by the commit that introduced a dummy RTE representing the output of the grouping step. I think that commit prevents one expression that matches the grouping item from being const-folded or preprocessed, but it doesn't prevent setrefs.c from matching the expression to some other identical ones. It seems to me that simple Var expressions in a subquery's target list also need to retain their separate identity in order to match grouping set columns after subquery pullup, not just non-var expressions. Alternatively, is there a way to teach setrefs.c to match an expression to the expected one when there are multiple identical expressions? [1] https://postgr.es/m/CAEZATCWsKqCtZ=ud26-ggv3zht-hjs4okg43gcbhsayuwyf...@mail.gmail.com Thanks Richard
