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 |
+----+-------------+---------+-
I think that "explain select * from table where relation_title='xxxxxxxxx';" returns the rows of relation_title='xxxxxxxxx' by index. But it's small than the true num.