LP:1002139 - Unsorted result for query with ORDER BY if ref_or_null access is used

CREATE TABLE t1 (

  a INT,

  c INT,

  UNIQUE KEY a_c (a,c),

  KEY (a)) engine=myisam;

INSERT INTO t1 VALUES (1,10), (2,NULL), (2,10);

ANALYZE TABLE t1;

EXPLAIN

SELECT c FROM t1 WHERE a=2 AND (c=10 OR c IS NULL) ORDER BY c;

SELECT c FROM t1 WHERE a=2 AND (c=10 OR c IS NULL) ORDER BY c;

MariaDB [test]> SELECT c FROM t1 WHERE a=2 AND (c=10 OR c IS NULL) ORDER BY c;

+------+

| c    |

+------+

|   10 |

| NULL |

+------+