mysql全文索引（三）查询扩展

1. 查询扩展原理：
   1. 开启查询扩展（query expansion）时候，进行两次查询，第一次，查出用户给定的关键词对应的记录；第二次，用第一次查出的结果里的关键词，再去查一次，把两次的结果返回给用户
2. 实验

   1. mysql> select id,title,body from articles where match(title,body) against('fulltext' ) order by id asc;
      +----+-------------------------+---------------------------------+
      | id | title                   | body                            |
      +----+-------------------------+---------------------------------+
      |  8 | MySQL Full-Text Indexes | MySQL fulltext indexes use a .. |
      +----+-------------------------+---------------------------------+
      1 row in set (0.00 sec)
      
      mysql> select id,title,body from articles where match(title,body) against('fulltext' with query expansion ) order by id asc;
      +----+-------------------------+-------------------------------------+
      | id | title                   | body                                |
      +----+-------------------------+-------------------------------------+
      |  1 | MySQL Tutorial          | This database tutorial ...          |
      |  2 | How To Use MySQL        | After you went through a ...        |
      |  4 | MySQL vs. YourSQL       | When comparing databases ...        |
      |  5 | MySQL Security          | When configured properly, MySQL ... |
      |  7 | 1001 MySQL Tricks       | 1. Never run mysqld as root. 2. ... |
      |  8 | MySQL Full-Text Indexes | MySQL fulltext indexes use a ..     |
      +----+-------------------------+-------------------------------------+
      6 rows in set (0.00 sec)
      

      　第一次没有开启查询扩展，只有包含关键词：fulltext的记录8被查到；第二次，包含mysql，fulltext的记录全部被返回

3. 　注意：

   1. 运用这个功能，可能会搜到很多不相关的信息，因此，只有当keyword比较short的时候，才使用
4. 参考资料：https://dev.mysql.com/doc/refman/5.7/en/fulltext-query-expansion.html