为什么where中出现了is null还能使用索引?
mysql> explain select * from book where t is null;
+----+-------------+-------+------+---------------+---------+---------+-------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+---------+---------+-------+------+--------------------------+
| 1 | SIMPLE | book | ref | t_index | t_index | 9 | const | 1 | Using where; Using index |
+----+-------------+-------+------+---------------+---------+---------+-------+------+--------------------------+
1 row in set
mysql>
这是为什么呢,另外,索引失效的情况有哪些呢