null和not null索引失效与否主要与表中字段的设立有关系，分为相应的两种情况，当对不能是null的字段使用索引时，条件无论是null或者not null 索引都失效，当对能是null的字段使用索引时，条件无论是null或者not null 索引都生效.
以下是null字段走索引的一个例子：
(root@localhost)-[09:51:01]-[(none)]>create database test;
Query OK, 1 row affected (0.02 sec)
(root@localhost)-[09:51:09]-[(none)]>CREATE TABLE `test_null` (
-> `id` int(11) DEFAULT NULL,
-> `mark` varchar(20) DEFAULT NULL
-> ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
(root@localhost)-[09:51:26]-[(none)]>use test
Database changed
(root@localhost)-[09:51:27]-[test]>CREATE TABLE `test_null` (
-> `id` int(11) DEFAULT NULL,
-> `mark` varchar(20) DEFAULT NULL
-> ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
Query OK, 0 rows affected (0.37 sec)
(root@localhost)-[09:51:29]-[test]>delimiter //
(root@localhost)-[09:51:37]-[test]>DROP PROCEDURE IF EXISTS test_null;
-> create procedure test_null(in num int)
-> BEGIN
-> DECLARE i int;
-> set i=1;
-> while (i<num)
-> DO
-> if mod(i,10)!=0 then
-> insert into test_null values (i,concat('aaa',i));
-> else
-> insert into test_null values (null,concat('aaa',i));
-> end if;
-> set i=i+1;
-> END while;
-> END;
-> //
Query OK, 0 rows affected, 1 warning (0.00 sec)
Query OK, 0 rows affected (0.00 sec)
(root@localhost)-[09:51:38]-[test]>delimiter ;
(root@localhost)-[09:51:44]-[test]>call test_null(10000);
Query OK, 1 row affected (12.34 sec)
(root@localhost)-[09:52:03]-[test]>
(root@localhost)-[09:52:03]-[test]>
(root@localhost)-[09:52:03]-[test]>
(root@localhost)-[09:52:03]-[test]>
(root@localhost)-[09:52:03]-[test]>
(root@localhost)-[09:52:03]-[test]>select count(*) from test_null;
+----------+
| count(*) |
+----------+
| 9999 |
+----------+
1 row in set (0.00 sec)
(root@localhost)-[09:52:24]-[test]>explain SELECT * from test_null WHERE id is null;
+----+-------------+-----------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| 1 | SIMPLE | test_null | NULL | ALL | NULL | NULL | NULL | NULL | 10003 | 10.00 | Using where |
+----+-------------+-----------+------------+------+---------------+------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
(root@localhost)-[09:52:34]-[test]>create index idx_test_null on test_null(id);
Query OK, 0 rows affected (0.13 sec)
Records: 0 Duplicates: 0 Warnings: 0
(root@localhost)-[09:52:46]-[test]>explain SELECT * from test_null WHERE id is null;
+----+-------------+-----------+------------+------+---------------+---------------+---------+-------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+------+---------------+---------------+---------+-------+------+----------+-----------------------+
| 1 | SIMPLE | test_null | NULL | ref | idx_test_null | idx_test_null | 5 | const | 999 | 100.00 | Using index condition |
+----+-------------+-----------+------------+------+---------------+---------------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
(root@localhost)-[09:52:54]-[test]>
(root@localhost)-[09:52:54]-[test]>explain SELECT * from test_null WHERE id is not null;
+----+-------------+-----------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| 1 | SIMPLE | test_null | NULL | ALL | idx_test_null | NULL | NULL | NULL | 10003 | 89.97 | Using where |
+----+-------------+-----------+------------+------+---------------+------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
建议:
MySQL列中尽量避免NULL,应该指定列为NOT NULL，除非你想存储NULL。在MySQL中，含有空值的列很难进行查询优化。因为它们使得索引、索引的统计信息以及比较运算更加复杂。你应该用0、一个特殊的值或者一个空串代替空值。
