create table ten(a int);

insert into ten values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);

create table one_k(a int);

insert into one_k select A.a + B.a* 10 + C.a * 100 from ten A, ten B, ten C;

create table t1 (

  a int,

  b varchar(32),

  c int,

  key(a),

  key(b)

) engine=innodb;

# 100K NULLs

insert into t1 select

  null,

  null,

  A.a + 1000*B.a 

from

  one_k A,

  one_k B

where 

  B.a<100;

# 900 K non-NULLs

insert into t1 select

  A.a + 1000*B.a,

  A.a + 1000*B.a,

  A.a + 1000*B.a 

from

  one_k A,

  one_k B

where 

  B.a>=100;

explain select * from t1 force index (a) where a is not null ;

+------+-------------+-------+-------+---------------+------+---------+------+--------+-----------------------+

| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows   | Extra                 |

+------+-------------+-------+-------+---------------+------+---------+------+--------+-----------------------+

|    1 | SIMPLE      | t1    | range | a             | a    | 5       | NULL | 494308 | Using index condition |

+------+-------------+-------+-------+---------------+------+---------+------+--------+-----------------------+

explain select * from t1 force index (b) where b is not null ;

+------+-------------+-------+-------+---------------+------+---------+------+--------+-----------------------+

| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows   | Extra                 |

+------+-------------+-------+-------+---------------+------+---------+------+--------+-----------------------+

|    1 | SIMPLE      | t1    | range | b             | b    | 35      | NULL | 494308 | Using index condition |

+------+-------------+-------+-------+---------------+------+---------+------+--------+-----------------------+