jverhoeks opened a new issue, #21570:
URL: https://github.com/apache/datafusion/issues/21570
**Describe the bug**
When `GROUP BY ROLLUP` is used with a query that produces 0 input rows,
DataFusion returns 0 output rows. The SQL standard requires ROLLUP to always
produce the grand total row (with NULL grouping keys and aggregate results
computed over the empty set).
**To Reproduce**
```sql
CREATE TABLE test (category VARCHAR, value INT) AS VALUES ('a', 1);
-- With data: works correctly (returns 2 rows: group + grand total)
SELECT category, count(*) FROM test GROUP BY ROLLUP(category);
-- Returns: [('a', 1), (NULL, 1)] ✅ correct
-- With empty input: returns 0 rows instead of 1
SELECT category, count(*) FROM test WHERE value < 0 GROUP BY
ROLLUP(category);
-- Returns: [] ❌ should return [(NULL, 0)]
-- Without GROUP BY, empty input correctly returns 1 row:
SELECT count(*) FROM test WHERE value < 0;
-- Returns: [(0)] ✅ correct
```
**Expected behavior**
Per SQL standard (and PostgreSQL, Trino, Oracle behavior):
```sql
SELECT category, count(*) FROM test WHERE value < 0 GROUP BY
ROLLUP(category);
-- Should return: [(NULL, 0)]
```
The grand total grouping set (the empty grouping set that ROLLUP always
includes) should produce a row even when there are zero input rows, because
aggregate functions like `count(*)` have well-defined behavior on empty sets
(returns 0).
**Additional context**
- Tested on DataFusion 52.3.0
- Same behavior with `CUBE` and explicit `GROUPING SETS((), (category))`
- Regular `GROUP BY category` correctly returns 0 rows with empty input (no
grand total expected)
- This affects 6 TPC-DS queries (q18, q27, q36, q67, q70, q86) at small
scale factors where certain joins produce 0 matching rows
**Root cause (likely)**
DataFusion's grouped aggregation implementation skips producing output
batches when the hash table is empty (no input rows). For regular `GROUP BY`,
this is correct. For `ROLLUP`/`CUBE`/`GROUPING SETS` that include the empty
grouping set `()`, a row should always be produced for that set.
The fix would need to check if any grouping set is the empty set, and if so,
produce a row with NULL grouping keys and empty-set aggregate values (count=0,
sum=NULL, etc.) even when the hash table has no entries.
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