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索引优化

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索引优化

单表优化案例

创建表

  • 建表 SQL
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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 VARCHAR(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');
  • 表中的测试数据
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mysql> SELECT * FROM article;
+----+-----------+-------------+-------+----------+-------+---------+
| id | author_id | category_id | views | comments | title | content |
+----+-----------+-------------+-------+----------+-------+---------+
|  1 |         1 |           1 |     1 |        1 | 1     | 1       |
|  2 |         2 |           2 |     2 |        2 | 2     | 2       |
|  3 |         1 |           1 |     3 |        3 | 3     | 3       |
+----+-----------+-------------+-------+----------+-------+---------+
3 rows in set (0.00 sec)

查询案例

  • 查询category_id为1且comments 大于1的情况下，views最多的article_id。
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mysql> SELECT id, author_id FROM article WHERE category_id = 1 AND comments > 1 ORDER BY views DESC LIMIT 1;
+----+-----------+
| id | author_id |
+----+-----------+
|  3 |         1 |
+----+-----------+
1 row in set (0.00 sec)
  • 此时 article 表中只有一个主键索引
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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)
  • 使用 explain 分析 SQL 语句的执行效率：EXPLAIN SELECT id, author_id FROM article WHERE category_id = 1 AND comments > 1 ORDER BY views DESC LIMIT 1;
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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   | type | possible_keys | key  | key_len | ref  | rows | Extra                       |
+----+-------------+---------+------+---------------+------+---------+------+------+-----------------------------+
|  1 | SIMPLE      | article | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where; Using filesort |
+----+-------------+---------+------+---------------+------+---------+------+------+-----------------------------+
1 row in set (0.00 sec)
  • 结论：
    • 很显然，type是ALL，即最坏的情况。
    • Extra 里还出现了Using filesort，也是最坏的情况。
    • 优化是必须的。

开始优化：新建索引

  • 创建索引的 SQL 命令
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# ALTER TABLE article ADD INDEX idx_article_ccv('category_id', 'comments', 'views'); 
create index idx_article_ccv on article(category_id, comments, views);
  • 在 category_id 列、comments 列和 views 列上建立联合索引
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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

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         |           3 |     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)
  • 再次执行查询：type变成了range，这是可以忍受的。但是extra里使用Using filesort仍是无法接受的。
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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   | type  | possible_keys   | key             | key_len | ref  | rows | Extra                                 |
+----+-------------+---------+-------+-----------------+-----------------+---------+------+------+---------------------------------------+
|  1 | SIMPLE      | article | range | idx_article_ccv | idx_article_ccv | 8       | NULL |    1 | Using index condition; Using filesort |
+----+-------------+---------+-------+-----------------+-----------------+---------+------+------+---------------------------------------+
1 row in set (0.00 sec)
  • 分析：
    • 但是我们已经建立了索引，为啥没用呢？
    • 这是因为按照B+Tree索引的工作原理，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，因为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。
  • 将查询条件中的 comments > 1 改为 comments = 1 ，发现 Use filesort 神奇地消失了，从这点可以验证：范围后的索引会导致索引失效
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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   | type | possible_keys   | key             | key_len | ref         | rows | Extra       |
+----+-------------+---------+------+-----------------+-----------------+---------+-------------+------+-------------+
|  1 | SIMPLE      | article | ref  | idx_article_ccv | idx_article_ccv | 8       | const,const |    1 | Using where |
+----+-------------+---------+------+-----------------+-----------------+---------+-------------+------+-------------+
1 row in set (0.00 sec)

删除索引

  • 删除索引的 SQL 指令
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DROP INDEX idx_article_ccv ON article;
  • 删除刚才创建的 idx_article_ccv 索引
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mysql> DROP INDEX idx_article_ccv ON article;
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

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)

再次创建索引

  • 创建索引的 SQL 指令
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# ALTER TABLE article ADD INDEX idx_article_ccv('category_id',  'views'); 
create index idx_article_ccv on article(category_id, views);
  • 由于 range 后（comments > 1）的索引会失效，这次我们建立索引时，直接抛弃 comments 列，先利用 category_id 和 views 的联合索引查询所需要的数据，再从其中取出 comments > 1 的数据（我觉着应该是这样的）
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mysql> create index idx_article_ccv on article(category_id, views);
Query OK, 0 rows affected (0.30 sec)
Records: 0  Duplicates: 0  Warnings: 0

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         |           3 |     NULL | NULL   |      | BTREE      |         |               |
| article |          1 | idx_article_ccv |            2 | views       | A         |           3 |     NULL | NULL   |      | BTREE      |         |               |
+---------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.00 sec)
  • 再次执行查询：可以看到，type变为了ref，Extra中的Using filesort也消失了，结果非常理想
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ysql> EXPLAIN SELECT id, author_id FROM article WHERE category_id = 1 AND comments > 1 ORDER BY views DESC LIMIT 1;
+----+-------------+---------+------+-----------------+-----------------+---------+-------+------+-------------+
| id | select_type | table   | type | possible_keys   | key             | key_len | ref   | rows | Extra       |
+----+-------------+---------+------+-----------------+-----------------+---------+-------+------+-------------+
|  1 | SIMPLE      | article | ref  | idx_article_ccv | idx_article_ccv | 4       | const |    2 | Using where |
+----+-------------+---------+------+-----------------+-----------------+---------+-------+------+-------------+
1 row in set (0.00 sec)
  • 为了不影响之后的测试，删除该表的 idx_article_ccv 索引
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mysql> DROP INDEX idx_article_ccv ON article;
Query OK, 0 rows affected (0.05 sec)
Records: 0  Duplicates: 0  Warnings: 0

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 (0.01 sec)

两表索引优化

创建表

  • 建表 SQL
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CREATE TABLE IF NOT EXISTS class(
	id INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
	card INT(10) UNSIGNED NOT NULL,
	PRIMARY KEY(id)
);

CREATE TABLE IF NOT EXISTS book(
	bookid INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
	card INT(10) UNSIGNED NOT NULL,
	PRIMARY KEY(bookid)
);

INSERT INTO class(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO class(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO class(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO class(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO class(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO class(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO class(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO class(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO class(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO class(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO class(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO class(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO class(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO class(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO class(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO class(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO class(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO class(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO class(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO class(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO class(card) VALUES(FLOOR(1+(RAND()*20)));

INSERT INTO book(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO book(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO book(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO book(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO book(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO book(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO book(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO book(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO book(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO book(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO book(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO book(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO book(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO book(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO book(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO book(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO book(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO book(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO book(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO book(card) VALUES(FLOOR(1+(RAND()*20)));
  • class 表中的测试数据
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mysql> select * from class;
+----+------+
| id | card |
+----+------+
|  1 |   12 |
|  2 |   13 |
|  3 |   12 |
|  4 |   17 |
|  5 |   11 |
|  6 |    3 |
|  7 |    1 |
|  8 |   16 |
|  9 |   17 |
| 10 |   16 |
| 11 |    9 |
| 12 |   17 |
| 13 |   18 |
| 14 |   16 |
| 15 |    7 |
| 16 |    8 |
| 17 |   19 |
| 18 |    9 |
| 19 |    6 |
| 20 |    5 |
| 21 |    6 |
+----+------+
21 rows in set (0.00 sec)
  • book 表中的测试数据
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mysql> select * from book;
+--------+------+
| bookid | card |
+--------+------+
|      1 |   16 |
|      2 |    1 |
|      3 |   17 |
|      4 |    3 |
|      5 |   20 |
|      6 |   12 |
|      7 |   18 |
|      8 |   13 |
|      9 |   13 |
|     10 |    4 |
|     11 |    1 |
|     12 |   13 |
|     13 |   20 |
|     14 |   20 |
|     15 |    1 |
|     16 |    2 |
|     17 |    9 |
|     18 |   16 |
|     19 |   14 |
|     20 |    2 |
+--------+------+
20 rows in set (0.00 sec)

查询案例

  • 实现两表的连接，连接条件是 class.card = book.card
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mysql> SELECT * FROM class LEFT JOIN book ON class.card = book.card;
+----+------+--------+------+
| id | card | bookid | card |
+----+------+--------+------+
|  1 |   12 |      6 |   12 |
|  2 |   13 |      8 |   13 |
|  2 |   13 |      9 |   13 |
|  2 |   13 |     12 |   13 |
|  3 |   12 |      6 |   12 |
|  4 |   17 |      3 |   17 |
|  5 |   11 |   NULL | NULL |
|  6 |    3 |      4 |    3 |
|  7 |    1 |      2 |    1 |
|  7 |    1 |     11 |    1 |
|  7 |    1 |     15 |    1 |
|  8 |   16 |      1 |   16 |
|  8 |   16 |     18 |   16 |
|  9 |   17 |      3 |   17 |
| 10 |   16 |      1 |   16 |
| 10 |   16 |     18 |   16 |
| 11 |    9 |     17 |    9 |
| 12 |   17 |      3 |   17 |
| 13 |   18 |      7 |   18 |
| 14 |   16 |      1 |   16 |
| 14 |   16 |     18 |   16 |
| 15 |    7 |   NULL | NULL |
| 16 |    8 |   NULL | NULL |
| 17 |   19 |   NULL | NULL |
| 18 |    9 |     17 |    9 |
| 19 |    6 |   NULL | NULL |
| 20 |    5 |   NULL | NULL |
| 21 |    6 |   NULL | NULL |
+----+------+--------+------+
28 rows in set (0.00 sec)
  • 使用 explain 分析 SQL 语句的性能，可以看到：驱动表是左表 class 表
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mysql> EXPLAIN SELECT * FROM class LEFT JOIN book ON class.card = book.card;
+----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                                              |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+
|  1 | SIMPLE      | class | ALL  | NULL          | NULL | NULL    | NULL |   21 | NULL                                               |
|  1 | SIMPLE      | book  | ALL  | NULL          | NULL | NULL    | NULL |   20 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+
2 rows in set (0.00 sec)
  • 结论：
    • type 有 All ，rows 为表中数据总行数，说明 class 和 book 进行了全表检索
    • 即每次 class 表对 book 表进行左外连接时，都需要在 book 表中进行一次全表检索

添加索引：在右表添加索引

  • 添加索引的 SQL 指令
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ALTER TABLE 'book' ADD INDEX Y ('card');
  • 在 book 的 card 字段上添加索引
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mysql> ALTER TABLE book ADD INDEX Y (card);
Query OK, 0 rows affected (0.30 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SHOW INDEX FROM book;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| book  |          0 | PRIMARY  |            1 | bookid      | A         |          20 |     NULL | NULL   |      | BTREE      |         |               |
| book  |          1 | Y        |            1 | card        | A         |          20 |     NULL | NULL   |      | BTREE      |         |               |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)
  • 测试结果：可以看到第二行的type变为了ref，rows也变成了优化比较明显。
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mysql> EXPLAIN SELECT * FROM class LEFT JOIN book ON class.card = book.card;
+----+-------------+-------+------+---------------+------+---------+-----------------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref             | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+-----------------+------+-------------+
|  1 | SIMPLE      | class | ALL  | NULL          | NULL | NULL    | NULL            |   21 | NULL        |
|  1 | SIMPLE      | book  | ref  | Y             | Y    | 4       | db01.class.card |    1 | Using index |
+----+-------------+-------+------+---------------+------+---------+-----------------+------+-------------+
2 rows in set (0.00 sec)
  • 分析：
    • 这是由左连接特性决定的。LEFT JOIN条件用于确定如何从右表搜索行，左边一定都有(左表示全量数据，圆规固定一脚是左表，右表转动需要建立索引)，所以右边是我们的关键点，一定需要建立索引。
    • 左表连接右表，则需要拿着左表的数据去右表里面查，索引需要在右表中建立索引

添加索引：在右表添加索引

  • 删除之前 book 表中的索引
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DROP INDEX Y ON book;
  • 在 class 表的 card 字段上建立索引
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ALTER TABLE class ADD INDEX X(card);
  • 再次执行左连接，凉凉~~~
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mysql> EXPLAIN SELECT * FROM class LEFT JOIN book ON class.card = book.card;
+----+-------------+-------+-------+---------------+------+---------+------+------+----------------------------------------------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra                                              |
+----+-------------+-------+-------+---------------+------+---------+------+------+----------------------------------------------------+
|  1 | SIMPLE      | class | index | NULL          | X    | 4       | NULL |   21 | Using index                                        |
|  1 | SIMPLE      | book  | ALL   | NULL          | NULL | NULL    | NULL |   20 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+-------+---------------+------+---------+------+------+----------------------------------------------------+
2 rows in set (0.00 sec)
  • 别怕，我们来执行右连接：可以看到第二行的type变为了ref，rows也变成了优化比较明显。
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mysql> EXPLAIN SELECT * FROM class RIGHT JOIN book ON class.card = book.card;
+----+-------------+-------+------+---------------+------+---------+----------------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref            | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+----------------+------+-------------+
|  1 | SIMPLE      | book  | ALL  | NULL          | NULL | NULL    | NULL           |   20 | NULL        |
|  1 | SIMPLE      | class | ref  | X             | X    | 4       | db01.book.card |    1 | Using index |
+----+-------------+-------+------+---------------+------+---------+----------------+------+-------------+
2 rows in set (0.00 sec)
  • 分析：
    • 这是因为RIGHT JOIN条件用于确定如何从左表搜索行，右边一定都有，所以左边是我们的关键点，一定需要建立索引。
    • class RIGHT JOIN book ：book 里面的数据一定存在于结果集中，我们需要拿着 book 表中的数据，去 class 表中搜索，所以索引需要建立在 class 表中
  • 为了不影响之后的测试，删除该表的 idx_article_ccv 索引
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mysql> DROP INDEX X ON class;
Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SHOW INDEX FROM class;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| class |          0 | PRIMARY  |            1 | id          | A         |          21 |     NULL | NULL   |      | BTREE      |         |               |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)

三表索引优化

创建表

  • 建表 SQL
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CREATE TABLE IF NOT EXISTS phone(
	phoneid INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
	card INT(10) UNSIGNED NOT NULL,
	PRIMARY KEY(phoneid)
)ENGINE=INNODB;

INSERT INTO phone(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card) VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card) VALUES(FLOOR(1+(RAND()*20)));
  • phone 表中的测试数据
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mysql> select * from phone;
+---------+------+
| phoneid | card |
+---------+------+
|       1 |    7 |
|       2 |    7 |
|       3 |   13 |
|       4 |    6 |
|       5 |    8 |
|       6 |    4 |
|       7 |   16 |
|       8 |    4 |
|       9 |   15 |
|      10 |    1 |
|      11 |   20 |
|      12 |   18 |
|      13 |    9 |
|      14 |    9 |
|      15 |   20 |
|      16 |   11 |
|      17 |   15 |
|      18 |    3 |
|      19 |    8 |
|      20 |   10 |
+---------+------+
20 rows in set (0.00 sec)

查询案例

  • 实现三表的连接查询：
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mysql> SELECT * FROM class LEFT JOIN book ON class.card = book.card LEFT JOIN phone ON book.card = phone.card;
+----+------+--------+------+---------+------+
| id | card | bookid | card | phoneid | card |
+----+------+--------+------+---------+------+
|  2 |   13 |      8 |   13 |       3 |   13 |
|  2 |   13 |      9 |   13 |       3 |   13 |
|  2 |   13 |     12 |   13 |       3 |   13 |
|  8 |   16 |      1 |   16 |       7 |   16 |
| 10 |   16 |      1 |   16 |       7 |   16 |
| 14 |   16 |      1 |   16 |       7 |   16 |
|  8 |   16 |     18 |   16 |       7 |   16 |
| 10 |   16 |     18 |   16 |       7 |   16 |
| 14 |   16 |     18 |   16 |       7 |   16 |
|  7 |    1 |      2 |    1 |      10 |    1 |
|  7 |    1 |     11 |    1 |      10 |    1 |
|  7 |    1 |     15 |    1 |      10 |    1 |
| 13 |   18 |      7 |   18 |      12 |   18 |
| 11 |    9 |     17 |    9 |      13 |    9 |
| 18 |    9 |     17 |    9 |      13 |    9 |
| 11 |    9 |     17 |    9 |      14 |    9 |
| 18 |    9 |     17 |    9 |      14 |    9 |
|  6 |    3 |      4 |    3 |      18 |    3 |
|  4 |   17 |      3 |   17 |    NULL | NULL |
|  9 |   17 |      3 |   17 |    NULL | NULL |
| 12 |   17 |      3 |   17 |    NULL | NULL |
|  1 |   12 |      6 |   12 |    NULL | NULL |
|  3 |   12 |      6 |   12 |    NULL | NULL |
|  5 |   11 |   NULL | NULL |    NULL | NULL |
| 15 |    7 |   NULL | NULL |    NULL | NULL |
| 16 |    8 |   NULL | NULL |    NULL | NULL |
| 17 |   19 |   NULL | NULL |    NULL | NULL |
| 19 |    6 |   NULL | NULL |    NULL | NULL |
| 20 |    5 |   NULL | NULL |    NULL | NULL |
| 21 |    6 |   NULL | NULL |    NULL | NULL |
+----+------+--------+------+---------+------+
30 rows in set (0.00 sec)
  • 使用 explain 分析 SQL 指令：
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mysql> EXPLAIN SELECT * FROM class LEFT JOIN book ON class.card = book.card LEFT JOIN phone ON book.card = phone.card;
+----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                                              |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+
|  1 | SIMPLE      | class | ALL  | NULL          | NULL | NULL    | NULL |   21 | NULL                                               |
|  1 | SIMPLE      | book  | ALL  | NULL          | NULL | NULL    | NULL |   20 | Using where; Using join buffer (Block Nested Loop) |
|  1 | SIMPLE      | phone | ALL  | NULL          | NULL | NULL    | NULL |   20 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+
3 rows in set (0.00 sec)
  • 结论：
    • type 有All ，rows 为表数据总行数，说明 class、 book 和 phone 表都进行了全表检索
    • Extra 中 Using join buffer ，表明连接过程中使用了 join 缓冲区

创建索引

  • 创建索引的 SQL 语句
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ALTER TABLE book ADD INDEX Y (card);
ALTER TABLE phone ADD INDEX Z (card);
  • 进行 LEFT JOIN ，永远都在右表的字段上建立索引
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mysql> ALTER TABLE book ADD INDEX Y (card);
Query OK, 0 rows affected (0.06 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SHOW INDEX FROM book;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| book  |          0 | PRIMARY  |            1 | bookid      | A         |          20 |     NULL | NULL   |      | BTREE      |         |               |
| book  |          1 | Y        |            1 | card        | A         |          20 |     NULL | NULL   |      | BTREE      |         |               |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)

mysql> ALTER TABLE phone ADD INDEX Z (card);
Query OK, 0 rows affected (0.05 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SHOW INDEX FROM phone;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| phone |          0 | PRIMARY  |            1 | phoneid     | A         |          20 |     NULL | NULL   |      | BTREE      |         |               |
| phone |          1 | Z        |            1 | card        | A         |          20 |     NULL | NULL   |      | BTREE      |         |               |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)
  • 执行查询：后2行的type都是ref，且总rows优化很好，效果不错。因此索引最好设置在需要经常查询的字段中。
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mysql> EXPLAIN SELECT * FROM class LEFT JOIN book ON class.card=book.card LEFT JOIN phone ON book.card = phone.card;
+----+-------------+-------+------+---------------+------+---------+-----------------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref             | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+-----------------+------+-------------+
|  1 | SIMPLE      | class | ALL  | NULL          | NULL | NULL    | NULL            |   21 | NULL        |
|  1 | SIMPLE      | book  | ref  | Y             | Y    | 4       | db01.class.card |    1 | Using index |
|  1 | SIMPLE      | phone | ref  | Z             | Z    | 4       | db01.book.card  |    1 | Using index |
+----+-------------+-------+------+---------------+------+---------+-----------------+------+-------------+
3 rows in set (0.00 sec)

Join 语句优化的结论

将 left join 看作是两层嵌套 for 循环

  1. 尽可能减少Join语句中的NestedLoop（嵌套勋魂）的循环总次数；

  2. 永远用小结果集驱动大的结果集（在大结果集中建立索引，在小结果集中遍历全表）；

    使用小表（商品类别）驱动大表（商品），因为小表是全量，大表是部分，所以需要在大表上建立索引。

  3. 优先优化NestedLoop的内层循环；

  4. 保证Join语句中被驱动表上Join条件字段已经被索引；

  5. 当无法保证被驱动表的Join条件字段被索引且内存资源充足的前提下，不要太吝惜JoinBuffer的设置；

索引失效

数据准备

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CREATE TABLE `staffs`(
`id` INT(10) PRIMARY KEY AUTO_INCREMENT,
`name` VARCHAR(24) NOT NULL DEFAULT '' COMMENT '姓名',
`age` INT(10) NOT NULL DEFAULT 0 COMMENT '年龄',
`pos` VARCHAR(20) NOT NULL DEFAULT '' COMMENT '职位',
`add_time` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '入职时间'
)COMMENT '员工记录表';

INSERT INTO `staffs`(`name`,`age`,`pos`) VALUES('Ringo', 18, 'manager');
INSERT INTO `staffs`(`name`,`age`,`pos`) VALUES('张三', 20, 'dev');
INSERT INTO `staffs`(`name`,`age`,`pos`) VALUES('李四', 21, 'dev');

/* 创建索引 */
CREATE INDEX idx_staffs_name_age_pos ON `staffs`(`name`,`age`,`pos`);

索引失效的情况

  • 全值匹配我最爱。
  • 最佳左前缀法则。
  • 不在索引列上做任何操作（计算、函数、(自动or手动)类型转换），会导致索引失效而转向全表扫描。
  • 索引中范围条件右边的字段会全部失效。
  • 尽量使用覆盖索引（只访问索引的查询，索引列和查询列一致），减少SELECT *。
  • MySQL在使用!=或者<>的时候无法使用索引会导致全表扫描。
  • is null、is not null也无法使用索引。
  • like以通配符开头%abc索引失效会变成全表扫描。
  • 字符串不加单引号索引失效。
  • 少用or，用它来连接时会索引失效。

最佳左前缀法则

案例

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/* 用到了idx_staffs_name_age_pos索引中的name字段 */
EXPLAIN SELECT * FROM `staffs` WHERE `name` = 'Ringo';

/* 用到了idx_staffs_name_age_pos索引中的name, age字段 */
EXPLAIN SELECT * FROM `staffs` WHERE `name` = 'Ringo' AND `age` = 18;

/* 用到了idx_staffs_name_age_pos索引中的name，age，pos字段 这是属于全值匹配的情况！！！*/
EXPLAIN SELECT * FROM `staffs` WHERE `name` = 'Ringo' AND `age` = 18 AND `pos` = 'manager';

/* 索引没用上，ALL全表扫描 */
EXPLAIN SELECT * FROM `staffs` WHERE `age` = 18 AND `pos` = 'manager';

/* 索引没用上，ALL全表扫描 */
EXPLAIN SELECT * FROM `staffs` WHERE `pos` = 'manager';

/* 用到了idx_staffs_name_age_pos索引中的name字段，pos字段索引失效 */
EXPLAIN SELECT * FROM `staffs` WHERE `name` = 'Ringo' AND `pos` = 'manager';

概念

最佳左前缀法则：如果索引是多字段的复合索引，要遵守最佳左前缀法则。指的是查询从索引的最左前列开始并且不跳过索引中的字段。

口诀：带头大哥不能死，中间兄弟不能断。

索引列上不计算

不在索引列上做任何操作（计算、函数、(自动or手动)类型转换），会导致索引失效而转向全表扫描。

案例

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# 现在要查询`name` = 'Ringo'的记录下面有两种方式来查询！

# 1、直接使用 字段 = 值的方式来计算
mysql> SELECT * FROM `staffs` WHERE `name` = 'Ringo';
+----+-------+-----+---------+---------------------+
| id | name  | age | pos     | add_time            |
+----+-------+-----+---------+---------------------+
|  1 | Ringo |  18 | manager | 2020-08-03 08:30:39 |
+----+-------+-----+---------+---------------------+
1 row in set (0.00 sec)

# 2、使用MySQL内置的函数
mysql> SELECT * FROM `staffs` WHERE LEFT(`name`, 5) = 'Ringo';
+----+-------+-----+---------+---------------------+
| id | name  | age | pos     | add_time            |
+----+-------+-----+---------+---------------------+
|  1 | Ringo |  18 | manager | 2020-08-03 08:30:39 |
+----+-------+-----+---------+---------------------+
1 row in set (0.00 sec)

我们发现以上两条SQL的执行结果都是一样的，但是执行效率有没有差距呢？？？

通过分析两条SQL的执行计划来分析性能。

https://gitee.com/lienhui68/picStore/raw/master/null/20201001122751.png

由此可见，在索引列上进行计算，会使索引失效。

口诀：索引列上不计算。

范围之后全失效

案例

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/* 用到了idx_staffs_name_age_pos索引中的name，age，pos字段 这是属于全值匹配的情况！！！*/
EXPLAIN SELECT * FROM `staffs` WHERE `name` = 'Ringo' AND `age` = 18 AND `pos` = 'manager';


/* 用到了idx_staffs_name_age_pos索引中的name，age字段，pos字段索引失效 */
EXPLAIN SELECT * FROM `staffs` WHERE `name` = '张三' AND `age` > 18 AND `pos` = 'dev';

查看上述SQL的执行计划

https://gitee.com/lienhui68/picStore/raw/master/null/20201001123049.png

由此可知，查询范围的字段使用到了索引，但是范围之后的索引字段会失效。

口诀：范围之后全失效。

覆盖索引尽量用

在写SQL的不要使用SELECT *，用什么字段就查询什么字段。

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/* 没有用到覆盖索引 */
EXPLAIN SELECT * FROM `staffs` WHERE `name` = 'Ringo' AND `age` = 18 AND `pos` = 'manager';

/* 用到了覆盖索引 */
EXPLAIN SELECT `name`, `age`, `pos` FROM `staffs` WHERE `name` = 'Ringo' AND `age` = 18 AND `pos` = 'manager';

https://gitee.com/lienhui68/picStore/raw/master/null/20201001123505.png

口诀：查询一定不用*。

不等会失效

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/* 会使用到覆盖索引 */
EXPLAIN SELECT `name`, `age`, `pos` FROM `staffs` WHERE `name` != 'Ringo';

/* 索引失效 全表扫描 */
EXPLAIN SELECT * FROM `staffs` WHERE `name` != 'Ringo';

is null，is not null 也无法使用索引

  • is null，is not null 会导致索引失效：key = null 表示索引失效
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mysql> EXPLAIN SELECT * FROM staffs WHERE name is null;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra            |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
1 row in set (0.00 sec)

mysql> EXPLAIN SELECT * FROM staffs WHERE name is not null;
+----+-------------+--------+------+-------------------------+------+---------+------+------+-------------+
| id | select_type | table  | type | possible_keys           | key  | key_len | ref  | rows | Extra       |
+----+-------------+--------+------+-------------------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | staffs | ALL  | index_staffs_nameAgePos | NULL | NULL    | NULL |    3 | Using where |
+----+-------------+--------+------+-------------------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

like

百分加右边

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/* 索引失效 全表扫描 */
EXPLAIN SELECT * FROM `staffs` WHERE `name` LIKE '%ing%';

/* 索引失效 全表扫描 */
EXPLAIN SELECT * FROM `staffs` WHERE `name` LIKE '%ing';

/* 使用索引范围查询 */
EXPLAIN SELECT * FROM `staffs` WHERE `name` LIKE 'Rin%';

口诀：like百分加右边。

百分加左边

如果一定要使用%like，而且还要保证索引不失效，那么使用覆盖索引来编写SQL。

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/* 使用到了覆盖索引 */
EXPLAIN SELECT `id` FROM `staffs` WHERE `name` LIKE '%in%';

/* 使用到了覆盖索引 */
EXPLAIN SELECT `name` FROM `staffs` WHERE `name` LIKE '%in%';

/* 使用到了覆盖索引 */
EXPLAIN SELECT `age` FROM `staffs` WHERE `name` LIKE '%in%';

/* 使用到了覆盖索引 */
EXPLAIN SELECT `pos` FROM `staffs` WHERE `name` LIKE '%in%';

/* 使用到了覆盖索引 */
EXPLAIN SELECT `id`, `name` FROM `staffs` WHERE `name` LIKE '%in%';

/* 使用到了覆盖索引 */
EXPLAIN SELECT `id`, `age` FROM `staffs` WHERE `name` LIKE '%in%';

/* 使用到了覆盖索引 */
EXPLAIN SELECT `id`,`name`, `age`, `pos` FROM `staffs` WHERE `name` LIKE '%in';

/* 使用到了覆盖索引 */
EXPLAIN SELECT `id`, `name` FROM `staffs` WHERE `pos` LIKE '%na';

/* 索引失效 全表扫描 */
EXPLAIN SELECT `name`, `age`, `pos`, `add_time` FROM `staffs` WHERE `name` LIKE '%in';

https://gitee.com/lienhui68/picStore/raw/master/null/20201001125652.png

口诀：覆盖索引保两边。

如果还需要其他列的内容，可以先把id查找出来，再用id去查找某一条具体的数据，这样就可以使用索引覆盖不回表了。

字符串不加单引号索引失效

  • 正常操作，索引没有失效
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mysql> SHOW INDEX FROM staffs;
+--------+------------+-------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table  | Non_unique | Key_name                | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+--------+------------+-------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| staffs |          0 | PRIMARY                 |            1 | id          | A         |           3 |     NULL | NULL   |      | BTREE      |         |               |
| staffs |          1 | index_staffs_nameAgePos |            1 | name        | A         |           3 |     NULL | NULL   |      | BTREE      |         |               |
| staffs |          1 | index_staffs_nameAgePos |            2 | age         | A         |           3 |     NULL | NULL   |      | BTREE      |         |               |
| staffs |          1 | index_staffs_nameAgePos |            3 | pos         | A         |           3 |     NULL | NULL   |      | BTREE      |         |               |
+--------+------------+-------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
4 rows in set (0.00 sec)

mysql> explain select * from staffs where name='2000';
+----+-------------+--------+------+-------------------------+-------------------------+---------+-------+------+-----------------------+
| id | select_type | table  | type | possible_keys           | key                     | key_len | ref   | rows | Extra                 |
+----+-------------+--------+------+-------------------------+-------------------------+---------+-------+------+-----------------------+
|  1 | SIMPLE      | staffs | ref  | index_staffs_nameAgePos | index_staffs_nameAgePos | 74      | const |    1 | Using index condition |
+----+-------------+--------+------+-------------------------+-------------------------+---------+-------+------+-----------------------+
1 row in set (0.00 sec)
  • 如果字符串忘记写 '' ，那么 mysql 会为我们进行隐式的类型转换，但凡进行了类型转换，索引都会失效
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mysql> explain select * from staffs where name=2000;
+----+-------------+--------+------+-------------------------+------+---------+------+------+-------------+
| id | select_type | table  | type | possible_keys           | key  | key_len | ref  | rows | Extra       |
+----+-------------+--------+------+-------------------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | staffs | ALL  | index_staffs_nameAgePos | NULL | NULL    | NULL |    3 | Using where |
+----+-------------+--------+------+-------------------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

使用 or 连接，会导致索引失效

5.5以上版本，使用or 是range查询

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mysql> SHOW INDEX FROM staffs;
+--------+------------+-------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table  | Non_unique | Key_name                | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+--------+------------+-------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| staffs |          0 | PRIMARY                 |            1 | id          | A         |           3 |     NULL | NULL   |      | BTREE      |         |               |
| staffs |          1 | index_staffs_nameAgePos |            1 | name        | A         |           3 |     NULL | NULL   |      | BTREE      |         |               |
| staffs |          1 | index_staffs_nameAgePos |            2 | age         | A         |           3 |     NULL | NULL   |      | BTREE      |         |               |
| staffs |          1 | index_staffs_nameAgePos |            3 | pos         | A         |           3 |     NULL | NULL   |      | BTREE      |         |               |
+--------+------------+-------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
4 rows in set (0.00 sec)

mysql> explain select * from staffs where name='z3' or name = 'v ';
+----+-------------+--------+------+-------------------------+------+---------+------+------+-------------+
| id | select_type | table  | type | possible_keys           | key  | key_len | ref  | rows | Extra       |
+----+-------------+--------+------+-------------------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | staffs | ALL  | index_staffs_nameAgePos | NULL | NULL    | NULL |    3 | Using where |
+----+-------------+--------+------+-------------------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

小结

假设index(a,b,c)

Where语句 索引是否被使用
where a = 3 Y，使用到a
where a = 3 and b = 5 Y，使用到a，b
where a = 3 and b = 5 Y，使用到a，b，c
where b = 3 或者 where b = 3 and c = 4 或者 where c = 4 N，没有用到a字段
where a = 3 and c = 5 使用到a，但是没有用到c，因为b断了
where a = 3 and b > 4 and c = 5 使用到a，b，但是没有用到c，因为c在范围之后
where a = 3 and b like ‘kk%’ and c = 4 Y，a，b，c都用到
where a = 3 and b like ‘%kk’ and c = 4 只用到a
where a = 3 and b like ‘%kk%’ and c = 4 只用到a
where a = 3 and b like ‘k%kk%’ and c = 4 Y，a，b，c都用到

like 后面以常量开头，比如 like ‘kk%’ 和 like ‘k%kk%’ ，可以理解为就是常量，注意区分范围查询，like ‘kk%‘的type虽然也是range，但是后面的索引列照样可以命中。

%kk，前面有%等于查所有，因为%表示任意。

kk%， 用到了索引

索引优化面试题

创建表

  • 建表 SQL
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create table test03(
    id int primary key not null auto_increment,
    c1 char(10),
    c2 char(10),
    c3 char(10),
    c4 char(10),
    c5 char(10)
);

insert into test03(c1,c2,c3,c4,c5) values ('a1','a2','a3','a4','a5');
insert into test03(c1,c2,c3,c4,c5) values ('b1','b2','b3','b4','b5');
insert into test03(c1,c2,c3,c4,c5) values ('c1','c2','c3','c4','c5');
insert into test03(c1,c2,c3,c4,c5) values ('d1','d2','d3','d4','d5');
insert into test03(c1,c2,c3,c4,c5) values ('e1','e2','e3','e4','e5');

create index idx_test03_c1234 on test03(c1,c2,c3,c4);
  • test03 表中的测试数据
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mysql> select * from test03;
+----+------+------+------+------+------+
| id | c1   | c2   | c3   | c4   | c5   |
+----+------+------+------+------+------+
|  1 | a1   | a2   | a3   | a4   | a5   |
|  2 | b1   | b2   | b3   | b4   | b5   |
|  3 | c1   | c2   | c3   | c4   | c5   |
|  4 | d1   | d2   | d3   | d4   | d5   |
|  5 | e1   | e2   | e3   | e4   | e5   |
+----+------+------+------+------+------+
5 rows in set (0.00 sec)
  • test03 表中的索引
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mysql> SHOW INDEX FROM test03;
+--------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table  | Non_unique | Key_name         | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+--------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| test03 |          0 | PRIMARY          |            1 | id          | A         |           5 |     NULL | NULL   |      | BTREE      |         |               |
| test03 |          1 | idx_test03_c1234 |            1 | c1          | A         |           5 |     NULL | NULL   | YES  | BTREE      |         |               |
| test03 |          1 | idx_test03_c1234 |            2 | c2          | A         |           5 |     NULL | NULL   | YES  | BTREE      |         |               |
| test03 |          1 | idx_test03_c1234 |            3 | c3          | A         |           5 |     NULL | NULL   | YES  | BTREE      |         |               |
| test03 |          1 | idx_test03_c1234 |            4 | c4          | A         |           5 |     NULL | NULL   | YES  | BTREE      |         |               |
+--------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
5 rows in set (0.00 sec)

问题：我们创建了复合索引idx_test03_c1234，根据以下SQL分析下索引使用情况？

  1. EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' AND c3='a3' AND c4='a4';

    • 即全值匹配

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      mysql> EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' AND c3='a3' AND c4='a4';
+----+-------------+--------+------+------------------+------------------+---------+-------------------------+------+-----------------------+
| id | select_type | table  | type | possible_keys    | key              | key_len | ref                     | rows | Extra                 |
+----+-------------+--------+------+------------------+------------------+---------+-------------------------+------+-----------------------+
|  1 | SIMPLE      | test03 | ref  | idx_test03_c1234 | idx_test03_c1234 | 124     | const,const,const,const |    1 | Using index condition |
+----+-------------+--------+------+------------------+------------------+---------+-------------------------+------+-----------------------+
1 row in set (0.00 sec)

  2. EXPLAIN SELECT * FROM test03 WHERE c4='a4' AND c3='a3' AND c2='a2' AND c1='a1';

    • mysql 优化器进行了优化，所以我们的索引都生效了

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      mysql> EXPLAIN SELECT * FROM test03 WHERE c4='a4' AND c3='a3' AND c2='a2' AND c1='a1';
+----+-------------+--------+------+------------------+------------------+---------+-------------------------+------+-----------------------+
| id | select_type | table  | type | possible_keys    | key              | key_len | ref                     | rows | Extra                 |
+----+-------------+--------+------+------------------+------------------+---------+-------------------------+------+-----------------------+
|  1 | SIMPLE      | test03 | ref  | idx_test03_c1234 | idx_test03_c1234 | 124     | const,const,const,const |    1 | Using index condition |
+----+-------------+--------+------+------------------+------------------+---------+-------------------------+------+-----------------------+
1 row in set (0.00 sec)

  3. EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' AND c3>'a3' AND c4='a4';

    • c3 列使用了索引进行排序，并没有进行查找，导致 c4 无法用索引进行查找

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      mysql> EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' AND c3>'a3' AND c4='a4'; 
+----+-------------+--------+-------+------------------+------------------+---------+------+------+-----------------------+
| id | select_type | table  | type  | possible_keys    | key              | key_len | ref  | rows | Extra                 |
+----+-------------+--------+-------+------------------+------------------+---------+------+------+-----------------------+
|  1 | SIMPLE      | test03 | range | idx_test03_c1234 | idx_test03_c1234 | 93      | NULL |    1 | Using index condition |
+----+-------------+--------+-------+------------------+------------------+---------+------+------+-----------------------+
1 row in set (0.00 sec)

  4. EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' AND c4>'a4' AND c3='a3';

    • mysql 优化器进行了优化，所以我们的索引都生效了，在 c4 时进行了范围搜索

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      mysql> EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' AND c4>'a4' AND c3='a3'; 
+----+-------------+--------+-------+------------------+------------------+---------+------+------+-----------------------+
| id | select_type | table  | type  | possible_keys    | key              | key_len | ref  | rows | Extra                 |
+----+-------------+--------+-------+------------------+------------------+---------+------+------+-----------------------+
|  1 | SIMPLE      | test03 | range | idx_test03_c1234 | idx_test03_c1234 | 124     | NULL |    1 | Using index condition |
+----+-------------+--------+-------+------------------+------------------+---------+------+------+-----------------------+
1 row in set (0.00 sec)

  5. EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' AND c4='a4' ORDER BY c3;

    • c3 列将索引用于排序，而不是查找，c4 列没有用到索引

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      mysql> EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' AND c4='a4' ORDER BY c3; 
+----+-------------+--------+------+------------------+------------------+---------+-------------+------+------------------------------------+
| id | select_type | table  | type | possible_keys    | key              | key_len | ref         | rows | Extra                              |
+----+-------------+--------+------+------------------+------------------+---------+-------------+------+------------------------------------+
|  1 | SIMPLE      | test03 | ref  | idx_test03_c1234 | idx_test03_c1234 | 62      | const,const |    1 | Using index condition; Using where |
+----+-------------+--------+------+------------------+------------------+---------+-------------+------+------------------------------------+
1 row in set (0.00 sec)

  6. EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' ORDER BY c3;

    • 和上面一样

  7. EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' ORDER BY c4;

    • 因为索引建立的顺序和使用的顺序不一致，导致 mysql 动用了文件排序

    • 看到 Using filesort 就要知道：此句 SQL 必须优化

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      mysql> EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' ORDER BY c4; 
+----+-------------+--------+------+------------------+------------------+---------+-------------+------+----------------------------------------------------+
| id | select_type | table  | type | possible_keys    | key              | key_len | ref         | rows | Extra                                              |
+----+-------------+--------+------+------------------+------------------+---------+-------------+------+----------------------------------------------------+
|  1 | SIMPLE      | test03 | ref  | idx_test03_c1234 | idx_test03_c1234 | 62      | const,const |    1 | Using index condition; Using where; Using filesort |
+----+-------------+--------+------+------------------+------------------+---------+-------------+------+----------------------------------------------------+
1 row in set (0.00 sec)

  8. EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c5='a5' ORDER BY c2, c3;

    • 只用 c1 一个字段索引，但是c2、c3用于排序，无filesort

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      mysql> EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c5='a5' ORDER BY c2, c3; 
+----+-------------+--------+------+------------------+------------------+---------+-------+------+------------------------------------+
| id | select_type | table  | type | possible_keys    | key              | key_len | ref   | rows | Extra                              |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+------------------------------------+
|  1 | SIMPLE      | test03 | ref  | idx_test03_c1234 | idx_test03_c1234 | 31      | const |    1 | Using index condition; Using where |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+------------------------------------+
1 row in set (0.00 sec)

  9. EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c5='a5' ORDER BY c3, c2;

    • 出现了filesort，我们建的索引是1234，它没有按照顺序来，32颠倒了

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      mysql> EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c5='a5' ORDER BY c3, c2; 
+----+-------------+--------+------+------------------+------------------+---------+-------+------+----------------------------------------------------+
| id | select_type | table  | type | possible_keys    | key              | key_len | ref   | rows | Extra                                              |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+----------------------------------------------------+
|  1 | SIMPLE      | test03 | ref  | idx_test03_c1234 | idx_test03_c1234 | 31      | const |    1 | Using index condition; Using where; Using filesort |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+----------------------------------------------------+
1 row in set (0.00 sec)

  10. EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' ORDER BY c2, c3;

    • 用c1、c2两个字段索引，但是c2、c3用于排序，无filesort

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      mysql> EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' ORDER BY c2, c3; 
+----+-------------+--------+------+------------------+------------------+---------+-------------+------+------------------------------------+
| id | select_type | table  | type | possible_keys    | key              | key_len | ref         | rows | Extra                              |
+----+-------------+--------+------+------------------+------------------+---------+-------------+------+------------------------------------+
|  1 | SIMPLE      | test03 | ref  | idx_test03_c1234 | idx_test03_c1234 | 62      | const,const |    1 | Using index condition; Using where |
+----+-------------+--------+------+------------------+------------------+---------+-------------+------+------------------------------------+
1 row in set (0.00 sec)

  11. EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' AND c5='a5' ORDER BY c2, c3;

    • c5不是组合索引列，和 c5 没啥关系

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      mysql> EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' AND c5='a5' ORDER BY c2, c3; 
+----+-------------+--------+------+------------------+------------------+---------+-------------+------+------------------------------------+
| id | select_type | table  | type | possible_keys    | key              | key_len | ref         | rows | Extra                              |
+----+-------------+--------+------+------------------+------------------+---------+-------------+------+------------------------------------+
|  1 | SIMPLE      | test03 | ref  | idx_test03_c1234 | idx_test03_c1234 | 62      | const,const |    1 | Using index condition; Using where |
+----+-------------+--------+------+------------------+------------------+---------+-------------+------+------------------------------------+
1 row in set (0.00 sec)

  12. EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' AND c5='a5' ORDER BY c3, c2;

    • 注意查询条件 c2=‘a2’ ，我都把 c2 查出来了（c2 为常量），我还给它排序作甚，所以没有产生 filesort

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      mysql> EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c4='a4' GROUP BY c2, c3; 
+----+-------------+--------+------+------------------+------------------+---------+-------+------+------------------------------------+
| id | select_type | table  | type | possible_keys    | key              | key_len | ref   | rows | Extra                              |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+------------------------------------+
|  1 | SIMPLE      | test03 | ref  | idx_test03_c1234 | idx_test03_c1234 | 31      | const |    1 | Using index condition; Using where |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+------------------------------------+
1 row in set (0.00 sec)

  13. EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c4='a4' GROUP BY c2, c3;

    • 顺序为 1 2 3 ，没有产生文件排序

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      mysql> EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c4='a4' GROUP BY c2, c3; 
+----+-------------+--------+------+------------------+------------------+---------+-------+------+------------------------------------+
| id | select_type | table  | type | possible_keys    | key              | key_len | ref   | rows | Extra                              |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+------------------------------------+
|  1 | SIMPLE      | test03 | ref  | idx_test03_c1234 | idx_test03_c1234 | 31      | const |    1 | Using index condition; Using where |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+------------------------------------+
1 row in set (0.00 sec)

  14. EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c4='a4' GROUP BY c3, c2;

    • group by 表面上叫分组，分组之前必排序，group by 和 order by 在索引上的问题基本是一样的

    • Using temporary; Using filesort 两个都有

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      mysql> EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c4='a4' GROUP BY c3, c2; 
+----+-------------+--------+------+------------------+------------------+---------+-------+------+---------------------------------------------------------------------+
| id | select_type | table  | type | possible_keys    | key              | key_len | ref   | rows | Extra                                                               |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+---------------------------------------------------------------------+
|  1 | SIMPLE      | test03 | ref  | idx_test03_c1234 | idx_test03_c1234 | 31      | const |    1 | Using index condition; Using where; Using temporary; Using filesort |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+---------------------------------------------------------------------+
1 row in set (0.01 sec)

    • group的本质是排序，分组之前必排序，group by和order by排序的算法和优化的原则是一样的，区别是group by有having

    • group by报错是sql_mod配置需要改一下，full group 要求select只能有group by的字段，不能包含别的。

    • group by 基本上都需要进行排序，但凡使用不当，会有临时表产生

建议

  1. 对于单键索引，尽量选择针对当前query过滤性更好的索引
  2. 在选择组合索引的时候，当前query中过滤性最好的字段在索引字段顺序中，位置越靠左越好。
  3. 在选择组合索引的时候，尽量选择可以能包含当前query中的where子句中更多字段的索引
  4. 尽可能通过分析统计信息和调整query的写法来达到选择合适索引的目的

总结

索引优化

建与不建，如何建（组合>单值）

更新于 2020-06-04
 mysql
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