BUG #19710: Incorrect DELETE result after LEFT JOIN optimization

The following bug has been logged on the website:

Bug reference: 19710
Logged by: cl hl
Email address: 2320415112@qq.com
PostgreSQL version: 17.10
Operating system: Linux LAPTOP-2SQAVLB0 6.6.87.2-microsoft-standard-
Description:

## Description

On PostgreSQL 17.10, a `DELETE` statement containing an `EXISTS` subquery
with two `LEFT JOIN`s deletes a row even though the subquery predicate is
false.

The test contains one source row whose `c2` value is `sample_b`, while the
query compares it with the constant `sample_a`. Therefore, the `EXISTS`
condition should be false. However, PostgreSQL returns and deletes the
target row.

Tested version:

```text
PostgreSQL 17.10 (Debian 17.10-1.pgdg13+1) on x86_64-pc-linux-gnu
```

## How to reproduce

Run the following script in a new session. The transaction is rolled back at
the end, so it does not leave any objects behind.

```sql
BEGIN;

CREATE TEMP TABLE t1 (
c1 text,
c2 text
);

CREATE TEMP TABLE t2 (
c1 text,
c2 text,
UNIQUE (c2, c1)
);

CREATE TEMP TABLE t3 (
c1 integer PRIMARY KEY,
c2 integer
);

INSERT INTO t1 VALUES ('sample_key', 'sample_b');
INSERT INTO t3 VALUES (1, 0);

SAVEPOINT initial_state;

-- Test query
WITH t4 AS (
SELECT 'sample_a' AS c1
)
DELETE FROM t3
WHERE EXISTS (
SELECT 1
FROM t1
LEFT JOIN t2
ON t2.c1 = t1.c1
AND t2.c2 = 'sample_a'
LEFT JOIN t4 ON true
WHERE t1.c2 = t4.c1
)
RETURNING c1;

SELECT * FROM t3;

ROLLBACK TO SAVEPOINT initial_state;

-- Control query: only `=` is changed to `>=`.
WITH t4 AS (
SELECT 'sample_a' AS c1
)
DELETE FROM t3
WHERE EXISTS (
SELECT 1
FROM t1
LEFT JOIN t2
ON t2.c1 = t1.c1
AND t2.c2 >= 'sample_a'
LEFT JOIN t4 ON true
WHERE t1.c2 = t4.c1
)
RETURNING c1;

SELECT * FROM t3;

ROLLBACK;
```

Both statements run from the same state. Since `t2` is empty, changing `=`
to
`>=` cannot change the result of either `LEFT JOIN` for this data.

The issue can also be seen by comparing the execution plans:

```sql
EXPLAIN (COSTS OFF)
WITH t4 AS (
SELECT 'sample_a' AS c1
)
DELETE FROM t3
WHERE EXISTS (
SELECT 1
FROM t1
LEFT JOIN t2
ON t2.c1 = t1.c1
AND t2.c2 = 'sample_a'
LEFT JOIN t4 ON true
WHERE t1.c2 = t4.c1
)
RETURNING c1;

EXPLAIN (COSTS OFF)
WITH t4 AS (
SELECT 'sample_a' AS c1
)
DELETE FROM t3
WHERE EXISTS (
SELECT 1
FROM t1
LEFT JOIN t2
ON t2.c1 = t1.c1
AND t2.c2 >= 'sample_a'
LEFT JOIN t4 ON true
WHERE t1.c2 = t4.c1
)
RETURNING c1;
```

Observed plan for the test query (`=`):

```text
Delete on t3
InitPlan 1
-> Seq Scan on t1
-> Result
One-Time Filter: (InitPlan 1).col1
-> Seq Scan on t3
```

Observed plan for the control query (`>=`):

```text
Delete on t3
InitPlan 1
-> Hash Left Join
Hash Cond: (t1.c1 = t2.c1)
Filter: (t1.c2 = 'sample_a'::text)
-> Seq Scan on t1
-> Hash
-> Bitmap Heap Scan on t2
Recheck Cond: (c2 >= 'sample_a'::text)
-> Bitmap Index Scan on t2_c2_c1_key
Index Cond: (c2 >= 'sample_a'::text)
-> Result
One-Time Filter: (InitPlan 1).col1
-> Seq Scan on t3
```

## Expected behavior

The predicate inside the `EXISTS` subquery is logically equivalent to:

```text
t1.c2 = 'sample_a'
```

The only row in `t1` has `c2 = 'sample_b'`, and `t2` is empty. Therefore,
the `EXISTS` condition should be false.

Both `DELETE` statements should return no rows:

```text
c1
----
(0 rows)
```

After each statement, the target row should remain:

```text
c1 | c2
----+----
1 | 0
(1 row)
```

The plan should preserve or derive a restriction equivalent to:

```text
Filter: (c2 = 'sample_a'::text)
```

## Actual behavior

The test query using `=` returns and deletes `c1 = 1`:

```text
c1
----
1
(1 row)

DELETE 1
```

The following `SELECT` returns no rows:

```text
c1 | c2
----+----
(0 rows)
```

After restoring the same initial state, the control query using `>=` returns
no
rows and leaves `(1, 0)` in `t3`:

```text
c1
----
(0 rows)

DELETE 0

c1 | c2
----+----
1 | 0
(1 row)
```

The test-query plan scans `t1` without the required `c2 = 'sample_a'`
filter.
The control-query plan retains that filter. As a result, only the `=` form
makes
the `EXISTS` condition true and causes an incorrect persistent-state change.