roseduan opened a new issue, #2025:
URL: https://github.com/apache/cloudberry/issues/2025
### Apache Cloudberry version
main branch
### What happened
With the Postgres planner (`optimizer = off`), a query that groups by a
constant expression returns one row when the input table is empty. A `GROUP BY`
over zero input rows has zero groups, so the
correct answer is no rows.
GPORCA (`optimizer = on`) is not affected. Non-empty inputs are not
affected — the extra row only appears when the input is empty.
### What you think should happen instead
_No response_
### How to reproduce
```sql
CREATE TABLE g(c0 boolean) DISTRIBUTED BY (c0); -- left EMPTY
SET optimizer = off;
SELECT count(*) FROM g GROUP BY 'x'::text;
-- count
-- -------
-- 0 <-- WRONG, expected 0 rows
SELECT 1 FROM g GROUP BY (0.25)::MONEY HAVING count(*) = 0;
-- ?column?
-- ----------
-- 1 <-- WRONG, expected 0 rows
```
Expected in both cases: `(0 rows)`.
The plan shows the grouping key disappearing and the final aggregate
becoming a plain `Aggregate`, which always emits one row:
```
EXPLAIN (VERBOSE, COSTS OFF) SELECT count(*) FROM g GROUP BY 'x'::text;
Finalize Aggregate
Output: count(*), 'x'::text
-> Gather Motion 3:1 (slice1; segments: 3)
Output: (PARTIAL count(*))
-> Partial GroupAggregate
Output: PARTIAL count(*)
-> Seq Scan on public.g
```
**Workarounds**
- `SET optimizer = on;` (GPORCA), or
- `SET gp_enable_multiphase_agg = off;`
### Operating System
any
### Anything else
_No response_
### Are you willing to submit PR?
- [ ] Yes, I am willing to submit a PR!
### Code of Conduct
- [x] I agree to follow this project's [Code of
Conduct](https://github.com/apache/cloudberry/blob/main/CODE_OF_CONDUCT.md).
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