Mysql子查询分析一

首先介绍几个常用的优化分析命令工具，刚开始写博客不习惯写详细说明

• EXPLAIN或DESC(describe缩写)SQL执行分析计划工具
• EXPLAIN的EXTENDED选项能够在原有的EXPLAIN基础上提供“查询优化SQL”，执行EXPLAIN EXTENDED SELECT...之后使用SHOW WARNINGS命令查看Mysql对原SQL的优化
• 配置profiling来分析查询SQL的执行过程
  写道
  show variables like "%profil%";
  set profiling = 1;
  select @@profiling;
• show profiles;    //查看操作记录
• show profile for query 1;   //查看具体的操作分析过程

接下来进入正题，之前写的一个报表sql中包含了子查询，刚开始还没发现什么问题，最近发现数据量也就3、4千的样子查询速度竟然达到2S左右。

简化SQL:
SELECT
count(1)
FROM
tb_mpsqd AS sqd1
LEFT JOIN tb_customer AS cust1 ON sqd1.customer_id = cust1.customer_id
LEFT JOIN t_s_depart AS depart1 ON sqd1.sys_org_code = depart1.org_code
LEFT JOIN tb_mpzqsqd AS zqsqd1 ON sqd1.mpsqd_id = zqsqd1.mpsqd_id AND sqd1.delayed_times = zqsqd1.delayed_times
LEFT JOIN tb_mppgd AS pgd1 ON sqd1.mpsqd_id = pgd1.mpsqd_id
LEFT JOIN (
select group_concat(distinct(a.classify_name)) as classify_name, group_concat(b.pawn_name) as pawn_name, group_concat(b.stand_by separator ' ') as wp_stand_by, b.mppgd_id from tb_classify as a inner join tb_mppgd_base as b on a.classify_id = b.classify_id GROUP BY b.mppgd_id
) AS pgdbase1 ON pgdbase1.mppgd_id = pgd1.mppgd_id
执行计划如下：
+----+-------------+------------+--------+-----------------+--------------+---------+-------------------------+------+----------------+
| id | select_type | table      | type   | possible_keys   | key          | key_len | ref                     | rows | Extra          |
+----+-------------+------------+--------+-----------------+--------------+---------+-------------------------+------+----------------+
|  1 | PRIMARY     | sqd1       | ALL    | NULL            | NULL         | NULL    | NULL                    | 1329 |                |
|  1 | PRIMARY     | cust1      | eq_ref | PRIMARY         | PRIMARY      | 4       | tdperp.sqd1.customer_id |    1 | Using index    |
|  1 | PRIMARY     | depart1    | ALL    | NULL            | NULL         | NULL    | NULL                    |  104 |                |
|  1 | PRIMARY     | zqsqd1     | ref    | mpsqd_id        | mpsqd_id     | 4       | tdperp.sqd1.mpsqd_id    |    1 |                |
|  1 | PRIMARY     | pgd1       | ref    | mppgd_id_idx    | mppgd_id_idx | 4       | tdperp.sqd1.mpsqd_id    |    1 | Using index    |
|  1 | PRIMARY     |  | ALL    | NULL            | NULL         | NULL    | NULL                    | 1116 |                |
|  2 | DERIVED     | b          | ALL    | idx_classify_id | NULL         | NULL    | NULL                    | 1446 | Using filesort |
|  2 | DERIVED     | a          | eq_ref | PRIMARY         | PRIMARY      | 4       | tdperp.b.classify_id    |    1 | Using where    |
+----+-------------+------------+--------+-----------------+--------------+---------+-------------------------+------+----------------+

分析：

1. 首先此报表业务关联了7张表，涉及到一些子表的属性group_concat和group by操作
2. sql中存在子查询，通过执行计划看到有DERIVED的查询类型，存储在临时表中。还存在一个 Using filesort文件排序问题（group by引起）
3. 通过拆分查询，并没有发现任何影响速度的问题。只有当子查询与主查询整合时速度就变慢

通过简单的测试后定位问题主要在于子查询这里：

1. 当使用子查询时，mysql需要创建临时表来存储并处理子查询的数据（filesort），且在本例中子查询的结果集是大字符串内容

子查询优化——解决filesort的问题

SELECT group_concat(DISTINCT(select classify_name from tb_classify a where b.classify_id = a.classify_id)) AS classify_name,
group_concat(b.pawn_name) AS pawn_name,
group_concat(b.stand_by SEPARATOR ' ') AS wp_stand_by,
b.mppgd_id 
FROM tb_mppgd_base AS b GROUP BY b.mppgd_id;
+----+--------------------+-------+--------+---------------+--------------+---------+----------------------+------+-------------+
| id | select_type        | table | type   | possible_keys | key          | key_len | ref                  | rows | Extra       |
+----+--------------------+-------+--------+---------------+--------------+---------+----------------------+------+-------------+
|  1 | PRIMARY            | b     | index  | NULL          | mppgd_id_idx | 4       | NULL                 | 1446 |             |
|  2 | DEPENDENT SUBQUERY | a     | eq_ref | PRIMARY       | PRIMARY      | 4       | tdperp.b.classify_id |    1 | Using where |
+----+--------------------+-------+--------+---------------+--------------+---------+----------------------+------+-------------+

子查询优化——解决速度问题

SELECT
count(1)
FROM
tb_mpsqd AS sqd1
LEFT JOIN tb_customer AS cust1 ON sqd1.customer_id = cust1.customer_id
LEFT JOIN t_s_depart AS depart1 ON sqd1.sys_org_code = depart1.org_code
LEFT JOIN tb_mpzqsqd AS zqsqd1 ON sqd1.mpsqd_id = zqsqd1.mpsqd_id AND sqd1.delayed_times = zqsqd1.delayed_times
LEFT JOIN tb_mppgd AS pgd1 ON sqd1.mpsqd_id = pgd1.mpsqd_id
LEFT JOIN tb_mppgd_base AS b ON b.mppgd_id = pgd1.mppgd_id 
LEFT JOIN tb_classify AS c ON b.classify_id = c.classify_id 
GROUP BY sqd1.mpsqd_id;
+----+-------------+---------+--------+-----------------+-----------------+---------+--------------------------+------+-------------+
| id | select_type | table   | type   | possible_keys   | key             | key_len | ref                      | rows | Extra       |
+----+-------------+---------+--------+-----------------+-----------------+---------+--------------------------+------+-------------+
|  1 | SIMPLE      | sqd1    | index  | NULL            | PRIMARY         | 4       | NULL                     | 1329 |             |
|  1 | SIMPLE      | cust1   | eq_ref | PRIMARY         | PRIMARY         | 4       | tdperp.sqd1.customer_id  |    1 | Using index |
|  1 | SIMPLE      | depart1 | ref    | unique_org_code | unique_org_code | 195     | tdperp.sqd1.sys_org_code |    1 | Using index |
|  1 | SIMPLE      | zqsqd1  | ref    | mpsqd_id        | mpsqd_id        | 4       | tdperp.sqd1.mpsqd_id     |    1 |             |
|  1 | SIMPLE      | pgd1    | ref    | mppgd_id_idx    | mppgd_id_idx    | 4       | tdperp.sqd1.mpsqd_id     |    1 | Using index |
|  1 | SIMPLE      | b       | ref    | mppgd_id_idx    | mppgd_id_idx    | 4       | tdperp.pgd1.mppgd_id     |    1 |             |
|  1 | SIMPLE      | c       | eq_ref | PRIMARY         | PRIMARY         | 4       | tdperp.b.classify_id     |    1 | Using index |
+----+-------------+---------+--------+-----------------+-----------------+---------+--------------------------+------+-------------+

优化后速度从2000ms提升到300ms以下。

总结：

1. 子查询的使用需要慎用，在Mysql5.6之后的版本已经做了优化，可以不考虑。Mysql5.6之前的版本建议尽量把子查询替换为Join关联查询。
2. 尽量在join关联的属性上建立索引
3. 使用ORDER BY null来避免filesort