mysql 全文索引查找不出来

ngram_token_size 设置为 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; INSERT INTO articles (title) VALUES("a"); INSERT INTO articles (title) VALUES("b"); INSERT INTO articles (title) VALUES("f"); 

搜索 a，返回空。为什么？

select * FROM articles WHERE MATCH (title) AGAINST ('+a' IN BOOLEAN MODE) ; 

ps， 把 a 改成 b 或者 f 则能正常返回。