mysql执行SQL语句时报错:[Err] 3 - Error writing file '/tmp/MYP0G1B8' (Errcode: 28 - No space left

[SQL] SELECT f.prov as 字段1, MAX( CASE f.flag_name WHEN '字段1' THEN f.num ELSE 0 END ) AS '字段1', MAX( CASE f.flag_name WHEN '店员量' THEN f.num ELSE 0 END ) AS '字段1', MAX( CASE f.flag_name WHEN '字段1' THEN f.num ELSE 0 END ) AS '字段1' FROM 
( SELECT '字段1' AS flag_name, ( SELECT b.AREA_NAME FROM sc_area b WHERE b.AREA_CODE = a.privinceCode ) AS prov, COUNT(DISTINCT a.storeID) AS num FROM promotion a WHERE DATE_FORMAT(a.pushTime, '%Y') = '2018'GROUP BY a.privinceCode 
  UNION ALL SELECT '字段1' AS flag_name, (   SELECT b.AREA_NAME FROM sc_area b WHERE b.AREA_CODE = a.privinceCode ) AS prov, COUNT(DISTINCT a.staffID) AS num FROM promotion a WHERE DATE_FORMAT(a.pushTime, '%Y') = '2018' GROUP BY a.privinceCode 
  UNION ALL SELECT '字段1' AS flag_name, ( SELECT b.AREA_NAME FROM sc_area b WHERE b.AREA_CODE = a.privinceCode ) AS prov, COUNT(1) AS num FROM promotion a WHERE DATE_FORMAT(a.pushTime, '%Y') = '2018' AND a.pushSts = '1' GROUP BY a.privinceCode 
) AS f GROUP BY f.prov ;
[Err] 3 - Error writing file '/tmp/MYP0G1B8' (Errcode: 28 - No space left on device)

[root@testvm mnt]# mkdir mysql_tmp_dir
[root@testvm mnt]# ls
mysql_tmp_dir
[root@testvm mnt]# chmod 777 mysql_tmp_dir/  #必须修改权限,让启动mysql的账户是可读写的. 此处是在root下创建,也可以切换到mysql用户下,然后创建目录,这样默认mysql就有读写权限了.
[root@testvm mnt]# ls -ltr
total 4
drwxrwxrwx 2 root root 4096 Jul  5 17:49 mysql_tmp_dir

mysql> show variables like '%tmpdir%';
+-------------------+--------------------+
| Variable_name     | Value              |
+-------------------+--------------------+
| innodb_tmpdir     |                    |
| slave_load_tmpdir | /mnt/mysql_tmp_dir |
| tmpdir            | /mnt/mysql_tmp_dir |
+-------------------+--------------------+
3 rows in set (0.01 sec)

The path of the directory to use for creating temporary files. It might be useful if your default /tmp directory resides on a partition 
that is too small to hold temporary tables. This option accepts several paths that are used in round-robin fashion. Paths should be 
separated by colon characters (:) on Unix and semicolon characters (;) on Windows. If the MySQL server is acting as a replication slave, 
you should not set --tmpdir to point to a directory on a memory-based file system or to a directory that is cleared when the server host 
restarts. For more information about the storage location of temporary files, see Section B.5.3.5, “Where MySQL Stores Temporary Files”. 
A replication slave needs some of its temporary files to survive a machine restart so that it can replicate temporary tables or LOAD DATA
 INFILE operations. If files in the temporary file directory are lost when the server restarts, replication fails.