MYSQL的UNIQUE索引包含null值及验证

在oracle我们知道唯一索引是不记录null值的。

在mysql中却不一样，mysql的唯一索引是记录null值的。以下摘录5.6手册中的话

A

UNIQUE

index
creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. For all engines, a

UNIQUE

index
permits multiple

NULL

values
for columns that can contain

NULL

.
If you specify a prefix value for a column in a

UNIQUE

index,
the column values must be unique within the prefix.

大意是说mysql的unique索引可包含多个null值。

那在谓词条件中使用is not null时，优化器是否会选择走索引呢。

我们来验证一下：

create table test1(id int primary key,tt int unique) engine innodb;

创建一个表id为主键，tt为唯一

使用一个procedure声称数据

CREATE DEFINER=`root`@`localhost` PROCEDURE `impdata`()

BEGIN

declare i int default 0;

while i<10000 DO

set i=i+1;

if i%500=1 then

insert into test1(id,tt) values(i,null);

else

insert into test1(id,tt) values(i,i);

end if;

end while;

END

然后执行

call impdata();

创建索引

create unique index test1_unq_tt on test1(tt);

查看执行计划

mysql> explain select tt from test1 where tt is not null;

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

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

| id | select_type | table | type | possible_keys | key | key_len | ref | r

ows | Extra |

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

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

| 1 | SIMPLE | test1 | range | tt,TEST1_UNQ_TT
| tt | 5 | NULL | 9

979 | Using where; Using index |

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

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

1 row in set (0.00 sec)

可见mysql的unique的索引是包含并记录null值的