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MySQL学习（八）：SQL查询语句的用法和优化

2019-04-14 23:38 701 查看
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一、概述

  • MySQL的性能优化可以从机器硬件，如磁盘，内存等；MySQL服务器配置，如线程数，查询缓存等；MySQL的主从分离和分库分表等；SQL语句优化等。其中SQL语句优化是与日常开发密切相关的，而且也是MySQL优化中最重要的一个环节，因为MySQL服务器，机器等的资源是一定的，故当出现性能瓶颈时，首先需要排除是否为SQL执行问题，如通过开启MySQL慢日志统计执行慢的SQL，或者使用profile功能统计SQL执行涉及的CPU，内存，IO等资源开销。

  • 定位到存在性能问题的SQL之后，则可以通过explain命令来分析该SQL的执行情况，如索引使用，排序等，然后是针对该SQL进行优化，优化主要从查询涉及的表，WHERE条件与是否使用和高效使用了索引，以及是否存在子查询等方面展开。

  • 要进行SQL优化，首先需要理解SQL的执行过程，具体可以参考：SQL解析顺序与MySQL底层实现

  • 以下分析以用户表t_user和用户订单表t_order来分析：在t_order表的user_id列是引用t_user的id列的外键。订单表和订单清单条目表t_order_item，通过order_id来建立外键约束。

    mysql> show create table t_user;
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table  | Create Table                                                                                                                                                                                                                                                                                                                                   |
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t_user | CREATE TABLE `t_user` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(64) NOT NULL,
`password` varchar(64) NOT NULL,
`email` varchar(64) NOT NULL,
`phone` varchar(32) DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `name` (`name`),
UNIQUE KEY `email` (`email`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8 |
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> show create table t_order;
+---------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table   | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
+---------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t_order | CREATE TABLE `t_order` (
`order_id` int(11) NOT NULL AUTO_INCREMENT,
`user_id` int(11) NOT NULL,
`cost` double DEFAULT NULL,
`buy_date` date NOT NULL,
PRIMARY KEY (`order_id`),
KEY `idx_order_buy_date` (`order_id`,`buy_date`),
KEY `idx_user_id` (`user_id`),
KEY `idx_user_id_buy_date` (`user_id`,`buy_date`),
CONSTRAINT `user_refrence` FOREIGN KEY (`user_id`) REFERENCES `t_user` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8 |
+---------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> show create table t_order_item;
+--------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table        | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
+--------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t_order_item | CREATE TABLE `t_order_item` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`product_id` int(11) NOT NULL,
`price` double NOT NULL,
`num` double NOT NULL,
`order_id` int(20) NOT NULL,
`remark` varchar(64) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `idx_remark` (`remark`),
KEY `idx_num` (`num`),
KEY `order_reference` (`order_id`),
CONSTRAINT `order_reference` FOREIGN KEY (`order_id`) REFERENCES `t_order` (`order_id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8 |
+--------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)

二、拼表优化：FROM和JOIN

  • MySQL多表查询：FROM和JOIN的用法与性能优化

三、WHERE查询优化

  • WHERE查询条件优化是SQL语句优化最重要的一个环节，WHERE子句的优化主要从索引的利用和条件的顺序两个方面。

1. 索引的利用

  • 通过给查询列增加索引可以避免全表扫描加快数据检索速度，同时覆盖索引还可以避免回表查询，只通过索引即可返回所需要的数据，索引相关的内容可以参考：MySQL学习（七）：Innodb存储引擎索引的实现原理
  • 如果WHERE查询条件中的索引列使用方法不当，则会导致索引失效，从而进行全表扫描，以下来分析索引失效的情况：
    MySQL索引失效的六种场景与案例分析

2. 查询条件的顺序

  • WHERE的查询条件的顺序主要是针对联合索引而言，即联合索引遵循最左前戳匹配规则，故需要保证在where中列从左到右，如联合索引(a,b,c)，则需要保证where a=xx and b=xx（注意，如果是where b=xx and a=xx，也可以继续使用该联合索引），而如果是where b=xx，则无法使用索引。如下对t_order_item表建立了联合索引idx_product_id_buy_date：当同时包含product_id和num或者只包含product_id时，可以使用该联合索引，如果只包含num则无法使用联合索引。

    mysql> alter table t_order_item add index idx_product_id_num(product_id, num);
Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> explain select * from t_order_item where num=2 and product_id=1;
+----+-------------+--------------+------------+------+----------------------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table        | partitions | type | possible_keys              | key     | key_len | ref   | rows | filtered | Extra       |
+----+-------------+--------------+------------+------+----------------------------+---------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | t_order_item | NULL       | ref  | idx_num,idx_product_id_num | idx_num | 8       | const |    1 |   100.00 | Using where |
+----+-------------+--------------+------------+------+----------------------------+---------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> alter table t_order_item drop index idx_num;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> explain select * from t_order_item where num=2 and product_id=1;
+----+-------------+--------------+------------+------+--------------------+--------------------+---------+-------------+------+----------+-------+
| id | select_type | table        | partitions | type | possible_keys      | key                | key_len | ref         | rows | filtered | Extra |
+----+-------------+--------------+------------+------+--------------------+--------------------+---------+-------------+------+----------+-------+
|  1 | SIMPLE      | t_order_item | NULL       | ref  | idx_product_id_num | idx_product_id_num | 12      | const,const |    1 |   100.00 | NULL  |
+----+-------------+--------------+------------+------+--------------------+--------------------+---------+-------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from t_order_item where product_id=1;
+----+-------------+--------------+------------+------+--------------------+--------------------+---------+-------+------+----------+-------+
| id | select_type | table        | partitions | type | possible_keys      | key                | key_len | ref   | rows | filtered | Extra |
+----+-------------+--------------+------------+------+--------------------+--------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | t_order_item | NULL       | ref  | idx_product_id_num | idx_product_id_num | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+--------------+------------+------+--------------------+--------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from t_order_item where num=2;
+----+-------------+--------------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table        | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+--------------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | t_order_item | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    1 |   100.00 | Using where |
+----+-------------+--------------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

  • 针对单列索引的情况，如果where的and条件中的列都包含索引或者某些没有索引，都是由MySQL自行选择使用其中一个MySQL优化器认为效率最高的索引，如下：user_id，order_id, buy_date均包含索引，则MySQL选择使用了order_id这个主键索引：

    mysql> explain select * from t_order where user_id=1 and order_id>2 and buy_date=curdate();
+----+-------------+---------+------------+-------+--------------------------------------------------------------------------+---------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type  | possible_keys                                                            | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+-------+--------------------------------------------------------------------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | t_order | NULL       | range | PRIMARY,idx_order_buy_date,idx_user_id,idx_user_id_buy_date,idx_buy_date | PRIMARY | 4       | NULL |    3 |    64.00 | Using where |
+----+-------------+---------+------------+-------+--------------------------------------------------------------------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.01 sec)

四、子查询的使用和优化

  • SQL优化（三）：子查询和IN，EXISTS用法和优化方法

五、ORDER BY 排序优化

  • ORDER BY的排序优化主要是利用索引的有序性来进行排序，从而减少MySQL服务器的排序操作，因为在服务端进行排序通常需要额外的内存空间，通常通过sort_buffer_size来控制每个MySQL服务器线程的sort buffer的大小，如果内存空间不够，则需要通过磁盘文件来辅助。所以如果能利用索引的有序性来完成排序而可以提高性能。

  • 所以ORDER BY的优化就转变为避免索引失效的优化了，即尽可能使用主键进行排序；如果不能使用主键来排序，则对于order by的列加上索引，并且如果可以使用覆盖索引，则通过建立联合索引来实现直接从索引返回数据；对于联合索引需要注意最左前戳匹配规则。如下，查询某个用户的所有订单并且根据购买日期排序，由执行计划可知，使用了联合索引idx_user_id_buy_date。

    mysql> select * from t_order where user_id=2 order by buy_date;
+----------+---------+------+------------+
| order_id | user_id | cost | buy_date   |
+----------+---------+------+------------+
|        5 |       2 | 1000 | 2019-04-14 |
+----------+---------+------+------------+
1 row in set (0.00 sec)

mysql> explain select * from t_order where user_id=2 order by buy_date;
+----+-------------+---------+------------+------+----------------------------------+----------------------+---------+-------+------+----------+-----------------------+
| id | select_type | table   | partitions | type | possible_keys                    | key                  | key_len | ref   | rows | filtered | Extra                 |
+----+-------------+---------+------------+------+----------------------------------+----------------------+---------+-------+------+----------+-----------------------+
|  1 | SIMPLE      | t_order | NULL       | ref  | idx_user_id,idx_user_id_buy_date | idx_user_id_buy_date | 4       | const |    1 |   100.00 | Using index condition |
+----+-------------+---------+------------+------+----------------------------------+----------------------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

  • 如果存在联合索引，但是不遵循最左前戳规则，则无法使用索引来排序，如下将user_id和buy_date反过来则无法使用联合索引idx_user_id_buy_date了，由Using filesort可知需要在MySQL服务器进行排序。

    mysql> explain select * from t_order where buy_date=curdate() order by user_id;
+----+-------------+---------+------------+------+---------------+--------------+---------+-------+------+----------+---------------------------------------+
| id | select_type | table   | partitions | type | possible_keys | key          | key_len | ref   | rows | filtered | Extra                                 |
+----+-------------+---------+------------+------+---------------+--------------+---------+-------+------+----------+---------------------------------------+
|  1 | SIMPLE      | t_order | NULL       | ref  | idx_buy_date  | idx_buy_date | 3       | const |    1 |   100.00 | Using index condition; Using filesort |
+----+-------------+---------+------------+------+---------------+--------------+---------+-------+------+----------+---------------------------------------+
1 row in set, 1 warning (0.01 sec)

六、LIMIT分页优化

  • LIMIT分页查询优化主要是针对LIMIT index, count形式的SQL，即从index下标开始的count条记录，如 LIMIT 10000, 50，取出第10000到10050这50条记录，但是对于MySQL来说需要扫描前面的10000条记录。所以可以基于以下思路来优化：
1. 记录上一页的有序的最大ID

  • 记录上一页的最大ID，通常为递增的主键值，或者递增的索引列，则可以利用索引来进行过滤，主要是基于B+树索引的特性来快速过滤掉大部分数据，如下：普通的limit index, count为全表扫描，使用order_id列则可以使用主键索引。

    mysql> explain select * from t_order  limit 10000, 10;
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | t_order | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    4 |   100.00 | NULL  |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from t_order where order_id > 100000 order by order_id limit 10;
+----+-------------+---------+------------+-------+----------------------------+---------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type  | possible_keys              | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+-------+----------------------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | t_order | NULL       | range | PRIMARY,idx_order_buy_date | PRIMARY | 4       | NULL |    1 |   100.00 | Using where |
+----+-------------+---------+------------+-------+----------------------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
2. 子查询优化

  • 可以通过子查询来对表的索引进行查找，获取这个范围的id，从而避免对数据表进行扫描，然后在数据表中取出匹配的数据行，如下：由于一页数据通常较少，故子查询

    mysql> select * from t_order inner join  (select order_id from t_order limit 10000, 10) as b on t_order.order_id=b.order_id;
Empty set (0.02 sec)

mysql> explain select * from t_order inner join  (select order_id from t_order limit 10000, 10) as b on t_order.order_id=b.order_id;
+----+-------------+------------+------------+-------+----------------------------+--------------+---------+---------------------------+------+----------+-------------+
| id | select_type | table      | partitions | type  | possible_keys              | key          | key_len | ref                       | rows | filtered | Extra       |
+----+-------------+------------+------------+-------+----------------------------+--------------+---------+---------------------------+------+----------+-------------+
|  1 | PRIMARY     | t_order    | NULL       | ALL   | PRIMARY,idx_order_buy_date | NULL         | NULL    | NULL                      |    4 |   100.00 | NULL        |
|  1 | PRIMARY     | <derived2> | NULL       | ref   | <auto_key0>                | <auto_key0>  | 4       | easy_web.t_order.order_id |    2 |   100.00 | Using index |
|  2 | DERIVED     | t_order    | NULL       | index | NULL                       | idx_buy_date | 3       | NULL                      |    4 |   100.00 | Using index |
+----+-------------+------------+------------+-------+----------------------------+--------------+---------+---------------------------+------+----------+-------------+
3 rows in set, 1 warning (0.00 sec)

七、GROUP BY 分组优化

  • GROUP BY操作通常会进行排序操作，而通过GROUP BY一般是与聚集函数，如SUM，COUNT，MAX等来结合使用从而完成统计任务，故一般不需要进行排序，如下：统计每个用户今天的订单总金额：

    mysql> select user_id, SUM(cost) from t_order where buy_date=curdate() group by user_id;
+---------+-----------+
| user_id | SUM(cost) |
+---------+-----------+
|       2 |      1000 |
+---------+-----------+
1 row in set (0.01 sec)

  • 执行计划如下：由extra的 Using filesort 可知，需要在MySQL服务器进行排序，但是此时并不需要该排序操作。

    mysql> explain select user_id, SUM(cost) from t_order where buy_date=curdate() group by user_id;
+----+-------------+---------+------------+------+-----------------------------------------------+--------------+---------+-------+------+----------+--------------------------------------------------------+
| id | select_type | table   | partitions | type | possible_keys                                 | key          | key_len | ref   | rows | filtered | Extra                                                  |
+----+-------------+---------+------------+------+-----------------------------------------------+--------------+---------+-------+------+----------+--------------------------------------------------------+
|  1 | SIMPLE      | t_order | NULL       | ref  | idx_user_id,idx_user_id_buy_date,idx_buy_date | idx_buy_date | 3       | const |    1 |   100.00 | Using index condition; Using temporary; Using filesort |
+----+-------------+---------+------------+------+-----------------------------------------------+--------------+---------+-------+------+----------+--------------------------------------------------------+

  • 所以可以通过 ORDER BY NULL 来禁止排序操作，如下：extra不再包含Using filesort信息，故在MySQL服务器不再进行排序操作。

    mysql> explain select user_id, SUM(cost) from t_order where buy_date=curdate() group by user_id order by null;
+----+-------------+---------+------------+------+-----------------------------------------------+--------------+---------+-------+------+----------+----------------------------------------+
| id | select_type | table   | partitions | type | possible_keys                                 | key          | key_len | ref   | rows | filtered | Extra                                  |
+----+-------------+---------+------------+------+-----------------------------------------------+--------------+---------+-------+------+----------+----------------------------------------+
|  1 | SIMPLE      | t_order | NULL       | ref  | idx_user_id,idx_user_id_buy_date,idx_buy_date | idx_buy_date | 3       | const |    1 |   100.00 | Using index condition; Using temporary |
+----+-------------+---------+------------+------+-----------------------------------------------+--------------+---------+-------+------+----------+----------------------------------------+
1 row in set, 1 warning (0.00 sec)
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