（四）MySQL单表索引优化

建表语句：

CREATE TABLE IF NOT EXISTS `article` (
`id` INT(10) UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
`author_id` INT(10) UNSIGNED NOT NULL,
`category_id` INT(10) UNSIGNED NOT NULL,
`views` INT(10) UNSIGNED NOT NULL,
`comments` INT(10) UNSIGNED NOT NULL,
`title` VARBINARY(255) NOT NULL,
`content` TEXT NOT NULL
);

INSERT INTO `article`(`author_id`, `category_id`, `views`, `comments`, `title`, `content`) VALUES
(1, 1, 1, 1, '1', '1'),
(2, 2, 2, 2, '2', '2'),
(1, 1, 3, 3, '3', '3');

查询索引:

mysql> show index from article;
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table   | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| article |          0 | PRIMARY  |            1 | id          | A         |           3 |     NULL | NULL   |      | BTREE      |         |               |
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)

• Table
• Non_unique: 非唯一索引(0为唯一索引, 1 为非唯一索引)
• Key_name: 表示索引的名称
• Seq_in_index: 表示该字段在索引中的位置，单列索引改值该值为1，组合索引为每个字段在索引中定义的顺序
• Column_name: 列名
• Collation: 字符序的规则
• Cardinality: 基数
• Sub_part: 表示索引的长度
• Packed: 是否创建压缩的索引(1, 0 与 Default)
• Null: 表示该字段是否能为空值
• Index_type: 表示索引类型
• Comment: 注释
• Index_comment: 索引注释

用explain查询sql

mysql> explain select id, author_id from article where category_id = 1 and comments > 1 order by views DESC LIMIT 1; 
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-----------------------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                       |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-----------------------------+
|  1 | SIMPLE      | article | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where; Using filesort |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-----------------------------+
1 row in set, 1 warning (0.00 sec)

• 可以看到type为all(全表查询), Extra中有Using filesort(使用了外部索引排序, 而不是按照表内索引顺序进行读取)。
• 需要进行优化, 不然数据量大后就比较危险。

创建复合索引

mysql> create index idx_article_ccv on article(category_id,comments,views);
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

1.展示该表的索引

mysql> show index from article;
+---------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table   | Non_unique | Key_name        | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+---------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| article |          0 | PRIMARY         |            1 | id          | A         |           3 |     NULL | NULL   |      | BTREE      |         |               |
| article |          1 | idx_article_ccv |            1 | category_id | A         |           2 |     NULL | NULL   |      | BTREE      |         |               |
| article |          1 | idx_article_ccv |            2 | comments    | A         |           3 |     NULL | NULL   |      | BTREE      |         |               |
| article |          1 | idx_article_ccv |            3 | views       | A         |           3 |     NULL | NULL   |      | BTREE      |         |               |
+---------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
4 rows in set (0.00 sec)

2.再次使用explain查询sql

mysql> explain select id, author_id from article where category_id = 1 and comments > 1 order by views DESC LIMIT 1;
+----+-------------+---------+------------+-------+-----------------+-----------------+---------+------+------+----------+---------------------------------------+
| id | select_type | table   | partitions | type  | possible_keys   | key             | key_len | ref  | rows | filtered | Extra                                 |
+----+-------------+---------+------------+-------+-----------------+-----------------+---------+------+------+----------+---------------------------------------+
|  1 | SIMPLE      | article | NULL       | range | idx_article_ccv | idx_article_ccv | 8       | NULL |    1 |   100.00 | Using index condition; Using filesort |
+----+-------------+---------+------------+-------+-----------------+-----------------+---------+------+------+----------+---------------------------------------+
1 row in set, 1 warning (0.00 sec)

• 可以看到type从all变成了range, 但是依旧走的外部索引排序, 而不是按照表内索引顺序进行读取。
• 索引失效的原理:
  • 使用的是B+树
  • 先排序 category_id
  • 如果遇到相同的 category_id 则再排序 comments, 如果遇到相同的 comments 则再排序 views。
  • 当 comments 字段在联合索引里处于中间位置时, 因comments > 1 条件是一个范围值(所谓range)
  • MySQL 无法利用索引再对后面的 views 部分进行检索, 即range 类型查询字段后面的索引无效。

删除原先索引, 构建新索引
3.删除索引

drop index idx_article_ccv on article;

4.创建新索引(避开了有范围条件的comments, 从而不会造成索引失效)

create index idx_article_cv on article(category_id, views);

5.再次查看索引

mysql> show index from article;
+---------+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table   | Non_unique | Key_name       | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+---------+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| article |          0 | PRIMARY        |            1 | id          | A         |           3 |     NULL | NULL   |      | BTREE      |         |               |
| article |          1 | idx_article_cv |            1 | category_id | A         |           2 |     NULL | NULL   |      | BTREE      |         |               |
| article |          1 | idx_article_cv |            2 | views       | A         |           3 |     NULL | NULL   |      | BTREE      |         |               |
+---------+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.00 sec)

6.再次使用explain查看sql语句

explain select id, author_id from article where category_id = 1 and comments > 1 order by views DESC LIMIT 1;
+----+-------------+---------+------------+------+----------------+----------------+---------+-------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys  | key            | key_len | ref   | rows | filtered | Extra       |
+----+-------------+---------+------------+------+----------------+----------------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | article | NULL       | ref  | idx_article_cv | idx_article_cv | 4       | const |    2 |    33.33 | Using where |
+----+-------------+---------+------------+------+----------------+----------------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

• 可以看到type变为了ref, extra中的file sorting消失了。

转载：https://www.cnblogs.com/ronnieyuan/p/12160291.html