Re: Unexpected results when using GROUP BY GROUPING SETS and bind variables

Aner Perez <aner(dot)perez+pgsql-general(at)gmail(dot)com> writes:
> [ these queries don't give the same results: ]

> SELECT path[1], path[2], path[3], path, sum(value)
> FROM bind_group_by
> GROUP BY GROUPING SETS (
> (path[1], path[2], path[3], path),
> (path[1], path[2], path[3]),
> (path[1], path[2]),
> (path[1]),
> ()
> )
> ORDER BY 1, 2, 3, 4
> ;

> PREPARE prepared_group_by (int, int, int, int, int, int, int, int, int,
> int, int, int) AS
> SELECT path[$1], path[$2], path[$3], path, sum(value)
> FROM bind_group_by
> GROUP BY GROUPING SETS (
> (path[$4], path[$5], path[$6], path),
> (path[$7], path[$8], path[$9]),
> (path[$10], path[$11]),
> (path[$12]),
> ()
> )
> ORDER BY 1, 2, 3, 4
> ;
> EXECUTE prepared_group_by (1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 1);

I think you're kind of shooting yourself in the foot here by writing
a query with more than one possible interpretation. Since you have
GROUP BY items that are both the whole "path" array and specific
elements of it, it's not clear whether a targetlist reference such
as "path[1]" is meant to refer to the GROUP BY item "path[1]", or
to be a subscripting operator applied to the GROUP BY item "path".

It appears that the parser makes the first choice in your first
query, although that seems like an implementation detail that
I wouldn't care to bet on going forward.

In the second query, the parser is certainly not going to think that
"path[$1]" matches "path[$4]" or any of the other single-element
GROUP BY items, so it decides that it means subscripting the "path"
item. Now the result will be NULL for any grouping set that doesn't
include the "path" item.

I haven't gone through the results one-by-one, but just by eyeball
I think this is sufficient to explain the discrepancies.

(I note that the SQL standard only allows GROUP BY items to be
simple column references. Maybe that's not an arbitrary restriction
but is meant to forestall this sort of ambiguity? Hard to be sure.)

regards, tom lane