Question

我不是mysql的专家，我不明白为什么没有使用主要索引。

创建表脚本：

create table BVorgang(
    Id bigint NOT NULL,
    Status varchar(2) NOT NULL,
... 
    SysVersion int NOT NULL,
    SysUserIn varchar(32) NULL,
    SysUserUp varchar(32) NULL,
    SysStampIn datetime NULL,
    SysStampUp datetime NULL);
alter table BVorgang add constraint PK_BVorgang primary key (Id);
create index IND_BVorgang_Status on BVorgang (Status);

hibernate生成的查询：

select count(id) from BVorgang where id in (select vorgang1_.id from BVorgang vorgang1_ where vorgang1_.status in ('20' , '30' , '70' , '40' , '75') group by vorgang1_.id);

我有一个带有mysql数据库5.6的开发人员系统mac os，我得到以下和预期的结果：

mysql> explain extended select count(id) from BVorgang where id in (select vorgang1_.id from BVorgang vorgang1_ where vorgang1_.status in ('20' , '30' , '70' , '40' , '75') group by vorgang1_.id);
+----+-------------+-----------+--------+-----------------------------+---------------------+---------+--------------------+------+----------+-------------+
| id | select_type | table     | type   | possible_keys               | key                 | key_len | ref                | rows | filtered | Extra       |
+----+-------------+-----------+--------+-----------------------------+---------------------+---------+--------------------+------+----------+-------------+
|  1 | SIMPLE      | BVorgang  | index  | PRIMARY                     | IND_BVorgang_Status | 4       | NULL               | 5144 |   100.00 | Using index |
|  1 | SIMPLE      | vorgang1_ | eq_ref | PRIMARY,IND_BVorgang_Status | PRIMARY             | 8       | fmtest.BVorgang.Id |    1 |   100.00 | Using where |
+----+-------------+-----------+--------+-----------------------------+---------------------+---------+--------------------+------+----------+-------------+
2 rows in set, 1 warning (0,00 sec)

mysql> select count(id) from BVorgang where id in (select vorgang1_.id from BVorgang vorgang1_ where vorgang1_.status in ('20' , '30' , '70' , '40' , '75') group by vorgang1_.id);
+-----------+
| count(id) |
+-----------+
|      5074 |
+-----------+
1 row in set (0,02 sec)

另一台机器是ubuntu 12.04测试服务器（4核7 gb ram），安装了mysql 5.5。如果我在这台机器上运行查询，我会得到以下结果：

mysql> explain extended select count(id) from BVorgang where id in (select vorgang1_.id from BVorgang vorgang1_ where vorgang1_.status in ('20' , '30' , '70' , '40' , '75') group by vorgang1_.id);
+----+--------------------+-----------+-------+---------------------+---------------------+---------+------+------+----------+------------------------------------------+
| id | select_type        | table     | type  | possible_keys       | key                 | key_len | ref  | rows | filtered | Extra                                    |
+----+--------------------+-----------+-------+---------------------+---------------------+---------+------+------+----------+------------------------------------------+
|  1 | PRIMARY            | BVorgang  | index | NULL                | IND_BVorgang_Status | 4       | NULL | 2228 |   100.00 | Using where; Using index                 |
|  2 | DEPENDENT SUBQUERY | vorgang1_ | range | IND_BVorgang_Status | IND_BVorgang_Status | 4       | NULL | 2039 |   100.00 | Using where; Using index; Using filesort |
+----+--------------------+-----------+-------+---------------------+---------------------+---------+------+------+----------+------------------------------------------+
2 rows in set, 1 warning (0.00 sec)

mysql> select count(id) from BVorgang where id in (select vorgang1_.id from BVorgang vorgang1_ where vorgang1_.status in ('20' , '30' , '70' , '40' , '75') group by vorgang1_.id);
+-----------+
| count(id) |
+-----------+
|      2036 |
+-----------+
1 row in set (1.59 sec)

我不知道为什么主要索引不在第二台机器上使用。也许有人可以帮助我。

mysql中没有使用主索引

0 个答案: