mysql полнотекстовый поиск не может искать некоторый контент

Question

Это структура таблицы и конфигурация:

mysql> select @@ngram_token_size ;
+--------------------+
| @@ngram_token_size |
+--------------------+
|                  1 |
+--------------------+

CREATE TABLE articles (
      id INT UNSIGNED AUTO_INCREMENT NOT NULL PRIMARY KEY,
      title VARCHAR(200),
      FULLTEXT (title) WITH PARSER ngram
    ) ENGINE=InnoDB CHARACTER SET utf8mb4;

и вставка данных:

INSERT INTO articles (title) VALUES("a");
INSERT INTO articles (title) VALUES("b");
INSERT INTO articles (title) VALUES("c");

поиск "b" или "c" нормальный.

mysql> select * FROM  articles  WHERE   MATCH (title) AGAINST ('+b' IN BOOLEAN MODE) ;
+----+-------+
| id | title |
+----+-------+
|  2 | b     |
+----+-------+
1 row in set (0.00 sec)

но, поиск "a" возврат пуст, почему?

mysql> select * FROM  articles  WHERE   MATCH (title) AGAINST ('+a' IN BOOLEAN MODE) ;
Empty set (0.00 sec)