SYSTEM VERSIONING NOT/CASE rewrite drops TIMESTAMP(6) precision and filters current rows

1. MariaDB SYSTEM VERSIONING: `NOT` over a `CASE` comparison drops `TIMESTAMP(6)` precision in `WHERE`

Status: reproduced on MariaDB 13.0.2. Relevant code reviewed on current `main`; current-main runtime was not tested.

1. 1. Reproduction environment

• MariaDB source: `mariadb-13.0.2`, commit `620a4b5624222048fece7fe1789978ae0675cae2`
• Current `main` reviewed at `9d7ab2c0092df4bc73b64b915464aecf7e5fe6c4`
• Linux x86_64, Clang 14, trace-pc-guard coverage build

1. 1. Minimal reproducer

```sql
SET time_zone = '+08:00';

CREATE TABLE t (
id INT PRIMARY KEY,
v VARCHAR(8),
valid_from TIMESTAMP(6) GENERATED ALWAYS AS ROW START,
valid_to TIMESTAMP(6) GENERATED ALWAYS AS ROW END,
PERIOD FOR SYSTEM_TIME (valid_from, valid_to)
) WITH SYSTEM VERSIONING;

INSERT INTO t(id, v) VALUES (1, 'a'), (2, 'b');

SELECT id, valid_to,
(CASE WHEN TRUE THEN valid_to ELSE valid_to END) <> valid_to AS p,
NOT ((CASE WHEN TRUE THEN valid_to ELSE valid_to END) <> valid_to) AS not_p
FROM t;

SELECT COUNT FROM t
WHERE NOT ((CASE WHEN TRUE THEN valid_to ELSE valid_to END) <> valid_to);

EXPLAIN EXTENDED
SELECT * FROM t
WHERE NOT ((CASE WHEN TRUE THEN valid_to ELSE valid_to END) <> valid_to);
SHOW WARNINGS;
```

1. 1. Actual and expected results

Each inserted current row has `valid_to = 2106-02-07 14:28:15.999999`. In projection, `p` is `0` and `not_p` is `1`. Nevertheless, the `WHERE NOT (...)` count is `0`; the expected count is `2`.

Controls using the same predicate with `IS FALSE` or `= 0` both return `2` rows.

1. 1. EXPLAIN evidence

`SHOW WARNINGS` exposes the rewritten condition as:

```sql
valid_to = TIMESTAMP'2106-02-07 14:28:15.999999'
AND '2106-02-07 14:28:15' = valid_to
```

The injected current-row bound retains `.999999`, while the propagated equality is a seconds-only string literal. The rewritten condition therefore rejects the current rows.

1. 1. Root cause

`NOT (A <> B)` is normalized during parsing: `negate_expression()` invokes the expression's negation transformer, and `Item_func_ne::negated_item()` constructs equality. That rewrite is valid by itself. The wrong result occurs when optimizer equality/temporal constant propagation interacts with the system-versioned current-row sentinel and the `CASE` expression, producing the seconds-only equality visible in `EXPLAIN`.

Relevant current-main code:

• [`negate_expression()`](https://github.com/MariaDB/server/blob/9d7ab2c0092df4bc73b64b915464aecf7e5fe6c4/sql/sql_parse.cc#L10021-L10055)
• [`Item_func_ne` negation](https://github.com/MariaDB/server/blob/9d7ab2c0092df4bc73b64b915464aecf7e5fe6c4/sql/item_cmpfunc.cc#L6854-L6972)
• [system-versioning condition construction](https://github.com/MariaDB/server/blob/9d7ab2c0092df4bc73b64b915464aecf7e5fe6c4/sql/sql_select.cc#L1043-L1080)
• [temporal max-value handling](https://github.com/MariaDB/server/blob/9d7ab2c0092df4bc73b64b915464aecf7e5fe6c4/sql/field.cc#L5919-L5941)

The minimized reproducer contains no GIS expression or `NULLIF`; this is unrelated to the `ST_IsValid(NULL)` predicate issue.

1. 1. Impact

A read-only `SELECT` can silently omit valid current rows when a negated comparison over a `CASE` expression references a system-versioned `TIMESTAMP(6)` period end. Projection and filtering disagree. No write corruption has been demonstrated.

1. 1. Duplicate audit

No exact public duplicate was found for the combination of the system-versioned `TIMESTAMP(6)` row-end sentinel, negated comparison, and precision-losing equality propagation. MDEV-36440 is related to `NOT NOT`/`WHERE` mismatches but does not contain this temporal sentinel or precision-propagation trigger. MDEV-8766 and MDEV-8706 are prior temporal `Item_equal` propagation bugs with different reproducers.

1. 1. Discovery

Found by DBMS-StateFuzz while exercising typed, compositional MariaDB queries and three-valued-logic partitioning.

Credit: Zheng Wang, Yanjie Zhao, and Yiyang Liu.