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:
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