Wrong query plan for query with no PK

CREATE TABLE t3(

  pk int primary key,

  a varchar(10) NOT NULL,   

  e int(11) DEFAULT 0,   

  KEY (a) 

) ENGINE=ROCKSDB DEFAULT CHARSET=utf8;

insert into t3 values (1,1,1),(2,2,2);

explain select a from t3 where a <'zzz';

+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+

| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra       |

+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+

|  1 | SIMPLE      | t3    | range | a             | a    | 32      | NULL |    2 | Using where |

+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+

CREATE TABLE t4(

  pk int,

  a varchar(10) NOT NULL,

  e int(11) DEFAULT 0,   

  KEY (a)

) ENGINE=ROCKSDB DEFAULT CHARSET=utf8;

insert into t4 values (1,1,1),(2,2,2);

explain select a from t4 where a <'zzz';

+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+

| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra       |

+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+

|  1 | SIMPLE      | t4    | range | a             | a    | 32      | NULL |    1 | Using where |

+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+

CREATE TABLE t3(

  pk int primary key,

  a varchar(10) NOT NULL,   

  e int(11) DEFAULT 0,   

  KEY (a) 

) ENGINE=ROCKSDB DEFAULT CHARSET=utf8;

insert into t3 values (1,1,1),(2,2,2);

explain select a from t3 where a <'zzz';

+------+-------------+-------+-------+---------------+------+---------+------+------+-------------+

| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra       |

+------+-------------+-------+-------+---------------+------+---------+------+------+-------------+

|    1 | SIMPLE      | t3    | range | a             | a    | 32      | NULL |    1 | Using where |

+------+-------------+-------+-------+---------------+------+---------+------+------+-------------+

CREATE TABLE t4(

  pk int,

  a varchar(10) NOT NULL,

  e int(11) DEFAULT 0,   

  KEY (a)

) ENGINE=ROCKSDB DEFAULT CHARSET=utf8;

insert into t4 values (1,1,1),(2,2,2);

explain select a from t4 where a <'zzz';

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra                    |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

|    1 | SIMPLE      | t4    | index | a             | a    | 32      | NULL |    0 | Using where; Using index |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+