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。