在mysql查询的表上创建索引，涉及join，group / order-by，union等

Question

下面是在mysql进程列表中有10个"copying to tmp table"状态的慢查询的解释输出。

explain SELECT distinct
    (radgroupreply.groupname),
    count(distinct (radusergroup.username)) AS users
FROM
    radgroupreply
        LEFT JOIN
    radusergroup ON radgroupreply.groupname = radusergroup.groupname
WHERE
    (radgroupreply.groupname NOT LIKE 'FB-%' AND radgroupreply.groupname NOT LIKE '%Dropped%')
GROUP BY radgroupreply.groupname 
UNION SELECT distinct
    (radgroupcheck.groupname),
    count(distinct (radusergroup.username))
FROM
    radgroupcheck
        LEFT JOIN
    radusergroup ON radgroupcheck.groupname = radusergroup.groupname
WHERE
    (radgroupcheck.groupname NOT LIKE 'FB-%' AND radgroupcheck.groupname NOT LIKE '%Dropped%')
GROUP BY radgroupcheck.groupname
ORDER BY groupname asc;

+----+--------------+---------------+------+---------------+------+---------+------+-------+----------------------------------------------+
| id | select_type  | table         | type | possible_keys | key  | key_len | ref  | rows  | Extra                                        |
+----+--------------+---------------+------+---------------+------+---------+------+-------+----------------------------------------------+
|  1 | PRIMARY      | radgroupreply | ALL  | NULL          | NULL | NULL    | NULL |   456 | Using where; Using temporary; Using filesort |
|  1 | PRIMARY      | radusergroup  | ALL  | NULL          | NULL | NULL    | NULL | 10261 |                                              |
|  2 | UNION        | radgroupcheck | ALL  | NULL          | NULL | NULL    | NULL |   167 | Using where; Using temporary; Using filesort |
|  2 | UNION        | radusergroup  | ALL  | NULL          | NULL | NULL    | NULL | 10261 |                                              |
|NULL| UNION RESULT | <union1,2>    | ALL  | NULL          | NULL | NULL    | NULL |  NULL | Using filesort                               |
+----+--------------+---------------+------+---------------+------+---------+------+-------+----------------------------------------------+
5 rows in set (0.00 sec)

我无法理解这个查询来创建复合/单索引并进行优化，因为它有多个连接，分组依赖和类似操作。

Answer 1

以下是三个开始的观察。

1. select distinct是不必要的（该group by由此处理）。
2. left join是不必要的（ where子句将它们转换为内连接）。
3. UNION应该是UNION ALL 。 我怀疑你真的想要消除重复的开销。

因此，您可以将查询编写为：

SELECT rr.groupname, count(distinct rg.username) AS users  
FROM radgroupreply rr JOIN
     radusergroup rg
     ON rr.groupname = rg.groupname                                                  
WHERE rr.groupname NOT LIKE 'FB-%' AND rr.groupname NOT LIKE '%Dropped%'
GROUP BY rr.groupname 
UNION ALL
SELECT rc.groupname, count(rg.username)
FROM radgroupcheck rc JOIN
     radusergroup rg
     ON rc.groupname = rg.groupname                                 
WHERE rc.groupname NOT LIKE 'FB-%' AND rc.groupname NOT LIKE '%Dropped%'
GROUP BY rc.groupname
ORDER BY groupname asc;

此查询可以利用radusergroup(groupname)上的索引。 我猜测将使用rc(radusergroup)上的索引。

如果没有必要，我还建议您删除COUNT(DISTINCT)中的DISTINCT 。