MySQL高级-SQL优化- order by 优化（尽量使用覆盖索引、注意联合索引创建的规则、增大排序缓冲区大小）

news2024/11/24 16:24:41

文章目录

  • 0、order by优化原则
  • 1、Using filesort
  • 2、Using index
  • 3、连接数据库
  • 4、查看索引
  • 5、删除索引
  • 6、按照年龄进行排序
  • 7、执行计划 order by age
  • 8、执行计划 order by age,phone
  • 9、创建联合索引 (age,phone)
  • 10、再次执行计划 order by age
  • 11、再次执行计划 order by age,phone
  • 12、执行计划 order by age desc,phone desc
  • 13、执行计划 order by phone,age
  • 14、执行计划 order by age asc,phone desc
  • 15、Collation（校对）是用于定义字符如何比较和排序的
  • 16、创建索引联合索引 (age asc,phone desc)
  • 17、再次执行计划 order by age asc,phone desc
  • 18、再次执行计划 order by age asc,phone asc
  • 19、升序/降序联合索引结构图示:

0、order by优化原则

  • 根据排序字段建立合适的索引，多字段排序时，也遵循最左前缀法则。
  • 尽量使用覆盖索引
  • 多字段排序，一个升序一个降序，此时需要注意联合索引在创建时的规则（ASC/DESC）
  • 如果不可避免的出现filesort，大数据量排序时，可以适当增大排序缓冲区大小sort_buffer_size（默认256K）
mysql> show variables like 'sort_buffer_size';
+------------------+--------+
| Variable_name    | Value  |
+------------------+--------+
| sort_buffer_size | 262144 |
+------------------+--------+
1 row in set, 1 warning (0.01 sec)

mysql>

1、Using filesort

通过表的索引或全表扫描，读取满足条件的数据行，然后在排序缓冲区sort buffer中完成排序操作，所有不是通过索引直接返回排序结果的排序都叫FileSort排序

2、Using index

通过有序索引顺序扫描直接返回有序数据，这种情况即为using index，不需要额外排序，操作效率高。

3、连接数据库

C:\Users\dgq>mysql -uroot -p
Enter password: ******
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 8
Server version: 8.0.26 MySQL Community Server - GPL

Copyright (c) 2000, 2021, Oracle and/or its affiliates.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql>

4、查看索引

mysql> use mybatis-example;
Database changed
mysql> show index from tb_user;
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table   | Non_unique | Key_name             | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| tb_user |          0 | PRIMARY              |            1 | id          | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          0 | idx_user_phone       |            1 | phone       | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          0 | idx_user_phone_name  |            1 | phone       | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          0 | idx_user_phone_name  |            2 | name        | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            1 | profession  | A         |          16 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            2 | age         | A         |          22 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            3 | status      | A         |          24 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_email_5          |            1 | email       | A         |          23 |        5 |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_name        |            1 | name        | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
9 rows in set (0.02 sec)

mysql>

5、删除索引

mysql> drop index idx_user_phone_name on tb_user;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> drop index idx_user_phone on tb_user;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> drop index idx_user_name on tb_user;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show index from tb_user;
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table   | Non_unique | Key_name             | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| tb_user |          0 | PRIMARY              |            1 | id          | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            1 | profession  | A         |          16 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            2 | age         | A         |          22 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            3 | status      | A         |          24 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_email_5          |            1 | email       | A         |          23 |        5 |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
5 rows in set (0.00 sec)

mysql>

6、按照年龄进行排序

mysql> select id,age,phone from tb_user order by age;
+----+------+-------------+
| id | age  | phone       |
+----+------+-------------+
| 22 |   19 | 17799990021 |
| 23 |   20 | 17799990022 |
|  6 |   22 | 17799990005 |
|  1 |   23 | 17799990000 |
|  5 |   23 | 17799990004 |
|  7 |   24 | 17799990006 |
| 10 |   27 | 17799990009 |
| 11 |   27 | 17799990010 |
| 12 |   29 | 17799990011 |
| 24 |   29 | 17799990023 |
| 19 |   30 | 17799990018 |
| 16 |   31 | 17799990015 |
|  2 |   33 | 17799990001 |
|  3 |   34 | 17799990002 |
| 17 |   35 | 17799990016 |
|  8 |   38 | 17799990007 |
| 18 |   38 | 17799990017 |
| 15 |   40 | 17799990014 |
|  9 |   43 | 17799990008 |
| 14 |   43 | 17799990013 |
| 13 |   44 | 17799990012 |
| 20 |   51 | 17799990019 |
| 21 |   52 | 17799990020 |
|  4 |   54 | 17799990003 |
+----+------+-------------+
24 rows in set (0.00 sec)

mysql>

7、执行计划 order by age

mysql> explain select id,age,phone from tb_user order by age;
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra          |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+----------------+
|  1 | SIMPLE      | tb_user | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   24 |   100.00 | Using filesort |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+----------------+
1 row in set, 1 warning (0.00 sec)

mysql>

8、执行计划 order by age,phone

mysql> explain select id,age,phone from tb_user order by age,phone;
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra          |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+----------------+
|  1 | SIMPLE      | tb_user | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   24 |   100.00 | Using filesort |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+----------------+
1 row in set, 1 warning (0.00 sec)

mysql>

9、创建联合索引 (age,phone)

mysql> create index idx_user_age_phone on tb_user(age,phone);
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show index from tb_user;
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table   | Non_unique | Key_name             | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| tb_user |          0 | PRIMARY              |            1 | id          | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            1 | profession  | A         |          16 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            2 | age         | A         |          22 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            3 | status      | A         |          24 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_email_5          |            1 | email       | A         |          23 |        5 |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_age_phone   |            1 | age         | A         |          19 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_age_phone   |            2 | phone       | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
7 rows in set (0.01 sec)

mysql>

10、再次执行计划 order by age

mysql> explain select id,age,phone from tb_user order by age;
+----+-------------+---------+------------+-------+---------------+--------------------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type  | possible_keys | key                | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+-------+---------------+--------------------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | index | NULL          | idx_user_age_phone | 48      | NULL |   24 |   100.00 | Using index |
+----+-------------+---------+------------+-------+---------------+--------------------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql>

11、再次执行计划 order by age,phone

mysql> explain select id,age,phone from tb_user order by age,phone;
+----+-------------+---------+------------+-------+---------------+--------------------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type  | possible_keys | key                | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+-------+---------------+--------------------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | index | NULL          | idx_user_age_phone | 48      | NULL |   24 |   100.00 | Using index |
+----+-------------+---------+------------+-------+---------------+--------------------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql>

12、执行计划 order by age desc,phone desc

mysql> explain select id,age,phone from tb_user order by age desc,phone desc;
+----+-------------+---------+------------+-------+---------------+--------------------+---------+------+------+----------+----------------------------------+
| id | select_type | table   | partitions | type  | possible_keys | key                | key_len | ref  | rows | filtered | Extra                            |
+----+-------------+---------+------------+-------+---------------+--------------------+---------+------+------+----------+----------------------------------+
|  1 | SIMPLE      | tb_user | NULL       | index | NULL          | idx_user_age_phone | 48      | NULL |   24 |   100.00 | Backward index scan; Using index |
+----+-------------+---------+------------+-------+---------------+--------------------+---------+------+------+----------+----------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql>

Extra: 提供了关于MySQL如何解决查询的额外信息。Backward index scan表示MySQL正在向后扫描索引（因为您使用了DESC）。Using index表示MySQL只使用了索引来满足查询，而不需要回表到数据行。

13、执行计划 order by phone,age

mysql> explain select id,age,phone from tb_user order by phone,age;
+----+-------------+---------+------------+-------+---------------+--------------------+---------+------+------+----------+-----------------------------+
| id | select_type | table   | partitions | type  | possible_keys | key                | key_len | ref  | rows | filtered | Extra                       |
+----+-------------+---------+------------+-------+---------------+--------------------+---------+------+------+----------+-----------------------------+
|  1 | SIMPLE      | tb_user | NULL       | index | NULL          | idx_user_age_phone | 48      | NULL |   24 |   100.00 | Using index; Using filesort |
+----+-------------+---------+------------+-------+---------------+--------------------+---------+------+------+----------+-----------------------------+
1 row in set, 1 warning (0.00 sec)

mysql>

MySQL执行查询时使用了idx_user_age_phone索引，但它仍然需要对结果进行额外的文件排序操作（Using filesort）。这是因为虽然查询使用了索引，但索引的顺序（先age后phone）与查询中ORDER BY子句指定的顺序（先phone后age）不匹配。

  • Extra 中的 Using index; Using filesort 表示使用了索引，但由于 排序顺序 与 索引顺序 不完全一致，还需要额外的文件排序操作。
  • Extra: 额外的信息。Using index表示MySQL只使用了索引来满足查询，但Using filesort表示MySQL需要对结果进行额外的排序操作，因为索引的顺序与ORDER BY子句指定的顺序不匹配。

14、执行计划 order by age asc,phone desc

mysql> explain select id,age,phone from tb_user order by age asc,phone desc;
+----+-------------+---------+------------+-------+---------------+--------------------+---------+------+------+----------+-----------------------------+
| id | select_type | table   | partitions | type  | possible_keys | key                | key_len | ref  | rows | filtered | Extra                       |
+----+-------------+---------+------------+-------+---------------+--------------------+---------+------+------+----------+-----------------------------+
|  1 | SIMPLE      | tb_user | NULL       | index | NULL          | idx_user_age_phone | 48      | NULL |   24 |   100.00 | Using index; Using filesort |
+----+-------------+---------+------------+-------+---------------+--------------------+---------+------+------+----------+-----------------------------+
1 row in set, 1 warning (0.00 sec)

mysql>

索引是按照(age, phone)的顺序构建的，即首先按age排序，然后在age相同的情况下按phone排序。但是，由于您希望按phone降序排序，而索引中的phone部分是升序的，因此MySQL不能仅使用索引来满足ORDER BY子句的要求，而必须进行额外的排序步骤。

15、Collation（校对）是用于定义字符如何比较和排序的

mysql> show index from tb_user;
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table   | Non_unique | Key_name             | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| tb_user |          0 | PRIMARY              |            1 | id          | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            1 | profession  | A         |          16 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            2 | age         | A         |          22 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            3 | status      | A         |          24 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_email_5          |            1 | email       | A         |          23 |        5 |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_age_phone   |            1 | age         | A         |          19 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_age_phone   |            2 | phone       | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
7 rows in set (0.00 sec)

mysql>

在MySQL中，Collation（校对）是用于定义字符如何比较和排序的。当你看到SHOW INDEX结果中的Collation列为A时，这通常意味着使用了默认的字符集和校对规则。在MySQL中，A通常代表latin1_swedish_ci（对于latin1字符集）或类似的默认校对规则，但具体的含义可能会根据MySQL的版本和配置有所不同。

在latin1_swedish_ci中：

  • latin1 是字符集（character set），它支持西欧语言中的字符。
  • swedish_ci 是校对规则（collation），它定义了字符如何比较和排序。ci表示大小写不敏感（case-insensitive）。

在 MySQL 中，“Collation”（排序规则）用于定义字符数据的排序和比较方式。 “A” 通常表示升序排序（Ascending），意味着在比较和排序字符数据时，按照字母表顺序从小到大进行排列。

16、创建索引联合索引 (age asc,phone desc)

mysql> create index idx_user_age_pho_ad on tb_user(age asc,phone desc);
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show index from tb_user;
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table   | Non_unique | Key_name             | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| tb_user |          0 | PRIMARY              |            1 | id          | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            1 | profession  | A         |          16 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            2 | age         | A         |          22 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            3 | status      | A         |          24 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_email_5          |            1 | email       | A         |          23 |        5 |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_age_phone   |            1 | age         | A         |          19 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_age_phone   |            2 | phone       | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_age_pho_ad  |            1 | age         | A         |          19 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_age_pho_ad  |            2 | phone       | D         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
9 rows in set (0.01 sec)

mysql>

17、再次执行计划 order by age asc,phone desc

mysql> explain select id,age,phone from tb_user order by age asc,phone desc;
+----+-------------+---------+------------+-------+---------------+---------------------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type  | possible_keys | key                 | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+-------+---------------+---------------------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | index | NULL          | idx_user_age_pho_ad | 48      | NULL |   24 |   100.00 | Using index |
+----+-------------+---------+------------+-------+---------------+---------------------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql>

18、再次执行计划 order by age asc,phone asc

mysql> explain select id,age,phone from tb_user order by age asc,phone asc;
+----+-------------+---------+------------+-------+---------------+--------------------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type  | possible_keys | key                | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+-------+---------------+--------------------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | index | NULL          | idx_user_age_phone | 48      | NULL |   24 |   100.00 | Using index |
+----+-------------+---------+------------+-------+---------------+--------------------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql>

19、升序/降序联合索引结构图示:

在这里插入图片描述

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