Wrong results: a filter on a constant column keeps the row of an aggregate without GROUP BY #26545

Describe the bug

A filter on a constant column, such as k in SELECT 'all' AS k, count(*) ..., is applied below an aggregate without GROUP BY, so it no longer removes the aggregate's row. An aggregate without GROUP BY returns one row even for an empty input, so the query returns that row (count(*) = 0, max(v) = NULL) where it should return none.

To Reproduce

datafusion-cli -c "
CREATE TABLE t(a INT, v INT) AS VALUES (1, 10), (2, 20), (2, 30);
SELECT c FROM (SELECT 'all' AS k, count(*) AS c FROM t) WHERE k = 'none';
"

This currently gives:

+---+
| c |
+---+
| 0 |
+---+

The same happens with other aggregates, with an empty grouping set (ROLLUP, CUBE, GROUPING SETS (..., ())), and in a UNION ALL that labels its branches:

With a true filter (WHERE k = 'all'), or with a grouped aggregate (GROUP BY a), the results are right.

Expected behavior

No rows: the subquery's only row has k = 'all', so k = 'none' is false for it. SQLite 3.51 returns no rows for the count(*) and max(v) queries, and 3 for the UNION ALL (it has no ROLLUP).

Additional context

PushDownFilter moves the filter below the aggregate. In its Aggregate arm, a predicate is pushed when all of its columns are group keys, which holds for a predicate with no columns. EXPLAIN VERBOSE shows logical_plan after push_down_filter with Filter: Utf8("all") = Utf8("none") between the Aggregate and the TableScan.

#22926 made the physical filter pushdown keep filters above aggregates that return a row for an empty input. PushDownFilter has no such check.

Unlike #26503, #26505 and #26508, no correlated subquery is involved, and fixing PushDownFilter does not change their results.

Found with DataFusion CLI v55.1.0, main at d89a85d.

Willingness to contribute

I plan to contribute a fix for this bug independently