解读索引列中有null值会不会使索引失效

create table null_test(
 id int PRIMARY KEY,
 name VARCHAR(10),
 age VARCHAR(10),
 KEY inx_test_age(age),
 KEY inx_test_name(name)
)
 
insert into null_test values(1,'a','2');
insert into null_test values(2,'b','3');
insert into null_test values(3,'c','4');
insert into null_test values(4,'d','5');
insert into null_test values(5,null,'6');
insert into null_test values(6,null,'6');
insert into null_test values(7,null,'9');
insert into null_test values(8,'q',null);
insert into null_test values(9,'','5');
insert into null_test values(10,'','7');
insert into null_test values(11,'t','');

explain
select * from null_test where name is null;

explain 
select * from null_test where name is not null;

create table null_test2(
 id int PRIMARY KEY,
 name VARCHAR(10),
 age VARCHAR(10),
 KEY inx_test2_age(age),
 KEY inx_test2_name(name)
)
 
 
insert into null_test2 values(1,'a','2');
insert into null_test2 values(2,'b','3');
insert into null_test2 values(3,'c','4');
insert into null_test2 values(4,'d','5');
insert into null_test2 values(5,null,'6');
insert into null_test2 values(6,null,'6');
insert into null_test2 values(7,null,'9');
insert into null_test2 values(8,null,'6');
insert into null_test2 values(9,null,'6');
insert into null_test2 values(10,null,'9');
insert into null_test2 values(11,null,'9');
insert into null_test2 values(12,null,'6');
insert into null_test2 values(13,null,'6');
insert into null_test2 values(14,null,'9');

explain
select * from null_test2 where name is null;

explain 
select * from null_test2 where name is not null;

create table null_test3(
 id int PRIMARY KEY,
 name VARCHAR(10),
 age VARCHAR(10),
 KEY inx_test2_age(age),
 UNIQUE KEY inx_test2_name(name)
)
 
insert into null_test3 values(1,'a','2');
insert into null_test3 values(2,'b','3');
insert into null_test3 values(3,'c','4');
insert into null_test3 values(4,'d','5');
insert into null_test3 values(5,null,'6');
insert into null_test3 values(6,null,'6');
insert into null_test3 values(7,null,'9');
insert into null_test3 values(8,null,'6');
insert into null_test3 values(9,null,'6');
insert into null_test3 values(12,'q',null);
insert into null_test3 values(13,'','5');
insert into null_test3 values(10,'g','7');
insert into null_test3 values(11,'t','');

explain
select * from null_test3 where name is null;

explain
select NAME from null_test3 where name is null;

show variables like 'innodb_stats_method';
SET GLOBAL  innodb_stats_method=nulls_unequal;