sql where Column is null 索引是否起作用

转自https://www.cnblogs.com/rocker-pg/p/9908506.html

 -- 生成test_null_index表
CREATE TABLE `test_null_index` (
       `id` int(11) DEFAULT NULL,
       `mark` varchar(20) DEFAULT NULL,
       `name` varchar(11) DEFAULT NULL
      ) ENGINE=InnoDB DEFAULT CHARSET=utf8;



 -- 生成测试数据
create procedure test_null_index(in num int)
BEGIN
DECLARE i int;  
set i=1;  
while (i<num) 
DO 
  if mod(i,10)!=0 then 
     insert into test_null_index values (i,concat('aaa',i),null);
   else
     insert into test_null_index values (null,concat('aaa',i),'bbb');
   end if;
set i=i+1;  
END while;  
END;

call test_null_index(10000);

 

然后执行  explain SELECT * from test_null_index WHERE id is null;

加索引 create index idx_test_null on test_null_index(id);执行  explain SELECT * from test_null_index WHERE id is null;

 

加索引create index idx_test_null2 on test_null_index(name);执行explain SELECT * from test_null_index WHERE id is null and name is null;

执行 explain SELECT * from test_null_index WHERE id is null or name is null;

 

结论：对一个字段做null判断，添加索引，索引是起作用的；对两个字段做null判断，and语句索引起作用；or语句索引没有作用，会放弃索引对整张表扫描。