More on GROUP BY

While looking at all these parsetrees I wonder why the hell
the GroupClause contains a complete copy of the TLE at all?
The planner depends on finding a corresponding entry in the
targetlist which should contain the same expression. At least
it needs an equal junk TLE. For the query

SELECT a, b FROM t1 GROUP BY b + 1;

the parser in fact creates 3 TLE's where the last one is a
junk result named "resjunk" for the "b + 1" expression and
the GroupClause contains a totally equal TLE.

Could someone explain that please?

Wouldn't it be better to have another field (resgroupno e.g.)
in the resdom which the GroupClause can reference? Then
changing the resno's or even replacing the entire expression
wouldn't hurt because make_subplanTargetList() could match
them this way and the expressions for the subplans can be
pulled out directly from the targetlist. And it would save
processing the group clauses in the rewriting because they
cannot contain Var nodes anymore and the entire list can be
ignored.

Jan

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