[I] [Bug] Wrong results: GROUP BY a constant expression returns one row for empty input [cloudberry]

["roseduan (via GitHub)" <gi...@apache.org>]

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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Re: [I] [Bug] Wrong results: GROUP BY a constant expression returns one row for empty input [cloudberry]

["Alena0704 (via GitHub)" <gi...@apache.org>]

Alena0704 commented on issue #2025:
URL: https://github.com/apache/cloudberry/issues/2025#issuecomment-5801209929

   Maybe useful: the same reproduction returns an incorrect result (0):
   ```
   set optimizer = off;
   create table t(c0 int);
   select 1 from t group by (0.5)::numeric;
   ```
   
   Building with `--enable-cassert` causes `FATAL ... ("false", File: "cdbgroupingpaths.c", Line: 1124)`.


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Re: [I] [Bug] Wrong results: GROUP BY a constant expression returns one row for empty input [cloudberry]

["tuhaihe (via GitHub)" <gi...@apache.org>]

tuhaihe closed issue #2025: [Bug]  Wrong results: GROUP BY a constant expression returns one row for empty input
URL: https://github.com/apache/cloudberry/issues/2025


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