[Bug] Incorrect result of COUNT(DISTINCT expr) FILTER (WHERE condition) in Cloudberry 2.1.0

[ryapandt]

Issue Body

Describe the bug

Apache Cloudberry returns incorrect results for COUNT(DISTINCT ...) FILTER (WHERE ...)
when gp_enable_multiphase_agg is enabled.

Two semantically equivalent aggregate expressions return different results.

Example:

COUNT(DISTINCT goods_gid)
    FILTER (WHERE invqty = 0)


COUNT(DISTINCT CASE WHEN invqty = 0 THEN goods_gid END)

According to SQL semantics, these two expressions should always return the same value.

However, with gp_enable_multiphase_agg=on, Cloudberry returns different results.

To Reproduce

Create test table:

CREATE TABLE test_cnt
(
    goods_gid int,
    invqty int
)
DISTRIBUTED BY (goods_gid);

Insert test data:

INSERT INTO test_cnt VALUES
(1,0),
(1,1),
(2,0),
(3,1);

Run query with multiphase aggregation enabled:

SET gp_enable_multiphase_agg=on;


SELECT
    COUNT(DISTINCT goods_gid)
        FILTER (WHERE invqty = 0) AS cnt1,


    COUNT(DISTINCT CASE
        WHEN invqty = 0 THEN goods_gid
    END) AS cnt2
FROM test_cnt;

query plan :

Aggregate  (cost=1.12..1.13 rows=1 width=16) (actual time=1.000..1.000 rows=1 loops=1)
  ->  Gather Motion 8:1  (slice1; segments: 8)  (cost=0.00..1.07 rows=5 width=8) (actual time=0.000..0.000 rows=5 loops=1)
        ->  Seq Scan on test_cnt  (cost=0.00..1.01 rows=1 width=8) (actual time=0.000..0.000 rows=2 loops=1)
Planning Time: 1.111 ms
  (slice0)    Executor memory: 31K bytes.
  (slice1)    Executor memory: 39K bytes avg x 8x(0) workers, 112K bytes max (seg0).
Memory used:  128000kB
Optimizer: Postgres query optimizer
Execution Time: 4.177 ms

Actual result:

cnt1 | cnt2
-----+-----
3 | 2

This result is incorrect.

Disable multiphase aggregation:

SET gp_enable_multiphase_agg=off;


SELECT
    COUNT(DISTINCT goods_gid)
        FILTER (WHERE invqty = 0) AS cnt1,


    COUNT(DISTINCT CASE
        WHEN invqty = 0 THEN goods_gid
    END) AS cnt2
FROM test_cnt;

Result:

cnt1 | cnt2
-----+-----
2 | 2

The result is correct.

Expected behavior

The following expressions should always return identical results:

COUNT(DISTINCT expression)
FILTER (WHERE condition)

and

COUNT(DISTINCT CASE WHEN condition THEN expression END)

The value should not depend on whether multiphase aggregation is enabled.

Actual behavior

When:

gp_enable_multiphase_agg=on

Cloudberry returns incorrect aggregate results.

When:

gp_enable_multiphase_agg=off

the result is correct.

Additional information

The issue is reproducible in our production environment.

The problem appears related to the combination of:

COUNT(DISTINCT ...)
FILTER clause
Multiphase aggregation
MPP aggregate execution

The workaround is to rewrite:

COUNT(DISTINCT col)
FILTER (WHERE condition)

as:

COUNT(DISTINCT CASE WHEN condition THEN col END)

or disable:

SET gp_enable_multiphase_agg=off;