BUG #19649: Qual pushdown into GROUP BY subqueries ignores non-equivalence-preserving references to grouping col

The following bug has been logged on the website:

Bug reference: 19649
Logged by: chunling qin
Email address: 303677365(at)qq(dot)com
PostgreSQL version: 18.6
Operating system: 86_64
Description:

When an outer WHERE/HAVING clause references a grouping column of a GROUP BY
(or DISTINCT) subquery through a type coercion (::text, CoerceViaIO) or a
function/operator wrapper (j->>0), the qual is pushed down below the
grouping node even though the reference applies a different equivalence
relation than the grouping does. Values that the grouping considers equal —
but whose text representations differ — get separated by the pushed-down
qual, splitting one group into two halves. This produces silently wrong
results: count(*) values change, a group can emit different group keys
depending on the WHERE, and rows are lost.

The simplest proof that something is wrong: the same subquery group answers
with two different group keys under two different outer WHERE clauses —
impossible under SQL semantics, since WHERE may only select subquery output
rows, never alter them.

CREATE TABLE t(id int primary key, j jsonb);
INSERT INTO t VALUES (1,'1'),(2,'1.0');
-- jsonb 1 = 1.0, so the table has exactly ONE jsonb group with count = 2

SELECT j, c FROM (SELECT j, count(*) c FROM t GROUP BY j) s;
-- 1 | 2 (baseline: one group)

SELECT j, c FROM (SELECT j, count(*) c FROM t GROUP BY j) s WHERE j::text =
'1';
-- 1 | 1 (WRONG: count changed by WHERE)

SELECT j, c FROM (SELECT j, count(*) c FROM t GROUP BY j) s WHERE j::text =
'1.0';
-- 1.0 | 1 (WRONG: the same group, different key)

SELECT j, c FROM (SELECT j, count(*) c FROM t GROUP BY j) s WHERE j =
'1'::jsonb;
-- 1 | 2 (control: same-eqop comparison is
correct)

```
hunt@(null)=# CREATE TABLE t(id int primary key, j jsonb);
INSERT INTO t VALUES (1,'1'),(2,'1.0');
-- jsonb 1 = 1.0, so the table has exactly ONE jsonb group with count = 2

SELECT j, c FROM (SELECT j, count(*) c FROM t GROUP BY j) s;
-- 1 | 2 (baseline: one group)

SELECT j, c FROM (SELECT j, count(*) c FROM t GROUP BY j) s WHERE j::text =
'1';
-- 1 | 1 (WRONG: count changed by WHERE)

SELECT j, c FROM (SELECT j, count(*) c FROM t GROUP BY j) s WHERE j::text =
'1.0';
-- 1.0 | 1 (WRONG: the same group, different key)

SELECT j, c FROM (SELECT j, count(*) c FROM t GROUP BY j) s WHERE j =
'1'::jsonb;
-- 1 | 2 (control: same-eqop comparison is
correct)
CREATE TABLE
INSERT 0 2
j | c
---+---
1 | 2
(1 row)

j | c
---+---
1 | 1
(1 row)

j | c
-----+---
1.0 | 1
(1 row)

j | c
---+---
1 | 2
(1 row)

hunt@(null)=# select version();
version

---------------------------------------------------------------------------------------------
------------------------------------------
PostgreSQL 20devel on x86_64-pc-linux-gnu, compiled by gcc (Tencent
Compiler 12.3.1.8) 12.3.
1 20230912 (TencentOS 12.3.1.8-6), 64-bit
(1 row)

```