mysql中未使用主索引

我不是mysql的专家，我不明白为什么不使用主索引

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);

我有一个开发人员系统mac os和mysql数据库5.6，我得到了以下和预期的结果：

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)

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)

另一台机器是安装了mysql 5.5的ubuntu 12.04测试服务器（4核7 gb ram）。如果在此计算机上运行查询，将得到以下意外结果：

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)

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)

---

我不知道为什么主索引没有在第二台机器上使用。也许有人能帮我。

尝试索引提示

从BVorgang使用索引（主）中选择计数（id），其中..

@Mihai:很遗憾，这是不可能的。查询是由hibernate生成的，我想做的最后一件事是更改在许多其他系统上正确运行的源代码。请发布生成此hibernate查询的代码以及正在计算的对象。这将帮助我们调试您的problem@JamesMassey：我认为您不需要hibernate代码，因为问题可以在纯SQL中重现。问题是——为什么mysql服务器会这样工作？它与hibernate无关，只是一个sql问题。