mysql> select count(*) from table where relation_title='xxxxxxxxx';
+----------+
| count(*) |
+----------+
|  1291958 |
+----------+

mysql> explain select *  from table where relation_title='xxxxxxxxx';
+----+-------------+---------+-
| id | select_type | rows    |
+----+-------------+---------+-
|  1 | SIMPLE      | 1274785 | 
+----+-------------+---------+-

我认为“请从“ relation_title =’xxxxxxxxx’的表中选择*”；通过索引返回relationship_title
=’xxxxxxxxx’的行。但这比实际数字小。