With derived_merge=on and join_cache_level=3, DECIMAL IS NULL predicate pushdown combined with BNLH hash join drops NULL rows: single-table returns NULL, JOIN rewrite returns empty set.

1. 1. 1. Problem Description
         With `optimizer_switch='derived_merge=on'` and `join_cache_level=3`, the original single-table query and the multi-table JOIN rewritten query return different results for the same logical condition `c1 IS NULL`. The single-table query returns one row `NULL`, while the JOIN rewritten query returns an empty set. It is suspected that after predicate pushdown, the optimizer/executor combines `r.c1 IS NULL` with the BNLH hash index build/probe on `r`'s covering index `k(id,c1)`. Since hash joins treat `NULL` keys with equality semantics, rows with `c1 IS NULL` are ignored or not correctly matched during the hash matching phase.

1. 1. 1. How to Reproduce
         Execute the following SQL:

```sql
DROP DATABASE IF EXISTS repro_mariadb814_db0_vp123_min;
CREATE DATABASE repro_mariadb814_db0_vp123_min;
USE repro_mariadb814_db0_vp123_min;

SET optimizer_switch='derived_merge=on';
SET join_cache_level=3;

– Original single table
CREATE TABLE s(
id BIGINT,
c0 BIT(8),
c1 DECIMAL(10,2)
) ENGINE=Aria;

– Reconstructed split-table structure
CREATE TABLE l(
id BIGINT PRIMARY KEY,
c0 BIT(8),
KEY k(c0,id)
) ENGINE=Aria;

CREATE TABLE r(
id BIGINT PRIMARY KEY,
c1 DECIMAL(10,2),
KEY k(id,c1)
) ENGINE=Aria;

INSERT INTO s VALUES
(1,NULL,NULL),
(2,2,2.01),
(3,3,3.01);

INSERT INTO l SELECT id,c0 FROM s;
INSERT INTO r SELECT id,c1 FROM s;

– Original single-table query
SELECT DISTINCT c1
FROM s
WHERE c1 IS NULL;

– Multi-table JOIN rewritten query
SELECT DISTINCT c1
FROM (
SELECT l.id, l.c0, r.c1
FROM l JOIN r ON l.id=r.id
) d
WHERE c1 IS NULL;
```

1. 1. 1. Expected Behavior
         The single-table query and the JOIN rewritten query should return the same result, i.e., both should return one row `NULL`.

1. 1. 1. Actual Behavior

• Single-table query: returns one row `NULL`.
• JOIN rewritten query: returns `Empty set`.

Single-table result:
```
------

------

------
```

JOIN rewritten result:
```
Empty set
```