1、单表索引优化
单表索引优化分析
创建表
建表 SQL
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
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。
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
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;
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
# 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
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。
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
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
DROP INDEX idx_article_ccv ON article;
删除刚才创建的 idx_article_ccv 索引
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
# 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）
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
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 | 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
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)
2、两表索引优化
两表索引优化分析：主外键
创建表
建表 SQL
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
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
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
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
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
ALTER TABLE 'book' ADD INDEX Y ('card');
在 book 的 card 字段上添加索引
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。
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，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
将左连接修改为右连接
删除之前 book 表中的索引
DROP INDEX Y ON book;
在 class 表的 card 字段上建立索引
ALTER TABLE class ADD INDEX X(card);
再次执行左连接，凉凉~~~
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。
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
3、三表索引优化
三表索引优化分析
创建表
建表 SQL
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
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：
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：
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
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
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。
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）；
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；
我的理解
1、使用小表驱动大表，这就相当于外层 for 循环的次数少，内层 for 循环的次数多
2、然后我们在大表中建立了索引，这样内层 for 循环的效率明显提高
3、综上，使用小表驱动大表，在大表中建立了索引
索引失效
索引失效（应该避免）
创建表
建表 SQL
CREATE TABLE staffs(
id INT PRIMARY KEY AUTO_INCREMENT,
`name` VARCHAR(24)NOT NULL DEFAULT'' COMMENT'姓名',
`age` INT 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'
)CHARSET utf8 COMMENT'员工记录表';
INSERT INTO staffs(`name`,`age`,`pos`,`add_time`) VALUES('z3',22,'manager',NOW());
INSERT INTO staffs(`name`,`age`,`pos`,`add_time`) VALUES('July',23,'dev',NOW());
INSERT INTO staffs(`name`,`age`,`pos`,`add_time`) VALUES('2000',23,'dev',NOW());
ALTER TABLE staffs ADD INDEX index_staffs_nameAgePos(`name`,`age`,`pos`);
staffs 表中的测试数据
mysql> select * from staffs;
+----+------+-----+---------+---------------------+
| id | name | age | pos | add_time |
+----+------+-----+---------+---------------------+
| 1 | z3 | 22 | manager | 2020-08-04 14:42:33 |
| 2 | July | 23 | dev | 2020-08-04 14:42:33 |
| 3 | 2000 | 23 | dev | 2020-08-04 14:42:33 |
+----+------+-----+---------+---------------------+
3 rows in set (0.00 sec)
staffs 表中的复合索引：name、age、pos
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)
1、索引失效准则
1、全值匹配我最爱
2、最佳左前缀法则：如果索引了多例，要遵守最左前缀法则。指的是查询从索引的最左前列开始并且不跳过索引中的列。
3、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
4、存储引擎不能使用索引中范围条件右边的列
5、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 *
6、mysql在使用不等于（!=或者<>）的时候无法使用索引会导致全表扫描
7、is null，is not null 也无法使用索引（早期版本不能走索引，后续版本应该优化过，可以走索引）
8、like以通配符开头（’%abc…’）mysql索引失效会变成全表扫描操作
9、字符串不加单引号索引失效
10、少用or，用它连接时会索引失效
最佳左匹配法则：带头大哥不能死，中间兄弟不能断
只有带头大哥 name 时
key = index_staffs_nameAgePos 表明索引生效
ref = const ：这个常量就是查询时的 ‘July’ 字符串常量
mysql> EXPLAIN SELECT * FROM staffs WHERE name = 'July';
+----+-------------+--------+------+-------------------------+-------------------------+---------+-------+------+-----------------------+
| 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)
带头大哥 name 带上小弟 age
key = index_staffs_nameAgePos 表明索引生效
ref = const,const：两个常量分别为 ‘July’ 和 23
mysql> EXPLAIN SELECT * FROM staffs WHERE name = 'July'AND age = 23;
+----+-------------+--------+------+-------------------------+-------------------------+---------+-------------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+-------------------------+-------------------------+---------+-------------+------+-----------------------+
| 1 | SIMPLE | staffs | ref | index_staffs_nameAgePos | index_staffs_nameAgePos | 78 | const,const | 1 | Using index condition |
+----+-------------+--------+------+-------------------------+-------------------------+---------+-------------+------+-----------------------+
1 row in set (0.00 sec)
带头大哥 name 带上小弟 age ，小弟 age 带上小小弟 pos
key = index_staffs_nameAgePos 表明索引生效
ref = const,const,const ：三个常量分别为 ‘July’、23 和 ‘dev’
mysql> EXPLAIN SELECT * FROM staffs WHERE name = 'July'AND age = 23 AND pos = 'dev';
+----+-------------+--------+------+-------------------------+-------------------------+---------+-------------------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+-------------------------+-------------------------+---------+-------------------+------+-----------------------+
| 1 | SIMPLE | staffs | ref | index_staffs_nameAgePos | index_staffs_nameAgePos | 140 | const,const,const | 1 | Using index condition |
+----+-------------+--------+------+-------------------------+-------------------------+---------+-------------------+------+-----------------------+
1 row in set (0.00 sec)
带头大哥 name 挂了
key = NULL 说明索引失效
ref = null 表示 ref 也失效
mysql> EXPLAIN SELECT * FROM staffs WHERE age = 23 AND pos = 'dev';
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | staffs | ALL | NULL | NULL | NULL | NULL | 3 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
带头大哥 name 没挂，小弟 age 跑了
key = index_staffs_nameAgePos 说明索引没有失效
ref = const 表明只使用了一个常量，即第二个常量（pos = ‘dev’）没有生效
mysql> EXPLAIN SELECT * FROM staffs WHERE name = 'July'AND pos = 'dev';
+----+-------------+--------+------+-------------------------+-------------------------+---------+-------+------+-----------------------+
| 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)
在索引列上进行计算，会导致索引失效，进而转向全表扫描
不对带头大哥 name 进行任何操作：key = index_staffs_nameAgePos 表明索引生效
mysql> EXPLAIN SELECT * FROM staffs WHERE name = 'July';
+----+-------------+--------+------+-------------------------+-------------------------+---------+-------+------+-----------------------+
| 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)
对带头大哥 name 进行操作：使用 LEFT 函数截取子串
key = NULL 表明索引生效
type = ALL 表明进行了全表扫描
mysql> EXPLAIN SELECT * FROM staffs WHERE LEFT(name,4) = 'July';
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | staffs | ALL | NULL | NULL | NULL | NULL | 3 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
范围之后全失效
精确匹配
type = ref 表示非唯一索引扫描，SQL 语句将返回匹配某个单独值的所有行。
key_len = 140 表明表示索引中使用的字节数
mysql> EXPLAIN SELECT * FROM staffs WHERE name = 'July'AND age = 23 AND pos = 'dev';
+----+-------------+--------+------+-------------------------+-------------------------+---------+-------------------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+-------------------------+-------------------------+---------+-------------------+------+-----------------------+
| 1 | SIMPLE | staffs | ref | index_staffs_nameAgePos | index_staffs_nameAgePos | 140 | const,const,const | 1 | Using index condition |
+----+-------------+--------+------+-------------------------+-------------------------+---------+-------------------+------+-----------------------+
1 row in set (0.00 sec)
将 age 改为范围匹配
type = range 表示范围扫描
key = index_staffs_nameAgePos 表示索引并没有失效
key_len = 78 ，ref = NULL 均表明范围搜索使其后面的索引均失效
mysql> EXPLAIN SELECT * FROM staffs WHERE name = 'July'AND age > 23 AND pos = 'dev';
+----+-------------+--------+-------+-------------------------+-------------------------+---------+------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+-------+-------------------------+-------------------------+---------+------+------+-----------------------+
| 1 | SIMPLE | staffs | range | index_staffs_nameAgePos | index_staffs_nameAgePos | 78 | NULL | 1 | Using index condition |
+----+-------------+--------+-------+-------------------------+-------------------------+---------+------+------+-----------------------+
1 row in set (0.00 sec)
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 *
SELECT * 的写法
mysql> EXPLAIN SELECT * FROM staffs WHERE name = 'July'AND age > 23 AND pos = 'dev';
+----+-------------+--------+-------+-------------------------+-------------------------+---------+------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+-------+-------------------------+-------------------------+---------+------+------+-----------------------+
| 1 | SIMPLE | staffs | range | index_staffs_nameAgePos | index_staffs_nameAgePos | 78 | NULL | 1 | Using index condition |
+----+-------------+--------+-------+-------------------------+-------------------------+---------+------+------+-----------------------+
1 row in set (0.00 sec)
覆盖索引的写法：Extra = Using where; Using index ，Using index 表示使用索引列进行查询，将大大提高查询的效率
mysql> EXPLAIN SELECT name, age, pos FROM staffs WHERE name = 'July'AND age = 23 AND pos = 'dev';
+----+-------------+--------+------+-------------------------+-------------------------+---------+-------------------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+-------------------------+-------------------------+---------+-------------------+------+--------------------------+
| 1 | SIMPLE | staffs | ref | index_staffs_nameAgePos | index_staffs_nameAgePos | 140 | const,const,const | 1 | Using where; Using index |
+----+-------------+--------+------+-------------------------+-------------------------+---------+-------------------+------+--------------------------+
1 row in set (0.00 sec)
覆盖索引中包含 range 条件：type = ref 并且 Extra = Using where; Using index ，虽然在查询条件中使用了 范围搜索，但是由于我们只需要查找索引列，所以无需进行全表扫描
mysql> EXPLAIN SELECT name, age, pos FROM staffs WHERE name = 'July'AND age > 23 AND pos = 'dev';
+----+-------------+--------+------+-------------------------+-------------------------+---------+-------+------+--------------------------+
| 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 where; Using index |
+----+-------------+--------+------+-------------------------+-------------------------+---------+-------+------+--------------------------+
1 row in set (0.00 sec)
mysql在使用不等于（!=或者<>）的时候无法使用索引会导致全表扫描
在使用 != 会 <> 时会导致索引失效：
key = null 表示索引失效
rows = 3 表示进行了全表扫描
mysql> EXPLAIN SELECT * FROM staffs WHERE name != 'July';
+----+-------------+--------+------+-------------------------+------+---------+------+------+-------------+
| 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)
mysql> EXPLAIN SELECT * FROM staffs WHERE name <> 'July';
+----+-------------+--------+------+-------------------------+------+---------+------+------+-------------+
| 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)
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
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 % 写应该最右边，在左边容易导致索引失效
staffs 表的索引关系
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)
like % 写在左边的情况
type = All ，rows = 3 表示进行了全表扫描
key = null 表示索引失效
mysql> EXPLAIN SELECT * FROM staffs WHERE name like '%July';
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | staffs | ALL | NULL | NULL | NULL | NULL | 3 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> EXPLAIN SELECT * FROM staffs WHERE name like '%July%';
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | staffs | ALL | NULL | NULL | NULL | NULL | 3 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
like % 写在右边的情况：key = index_staffs_nameAgePos 表示索引未失效
mysql> EXPLAIN SELECT * FROM staffs WHERE name like 'July%';
+----+-------------+--------+-------+-------------------------+-------------------------+---------+------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+-------+-------------------------+-------------------------+---------+------+------+-----------------------+
| 1 | SIMPLE | staffs | range | index_staffs_nameAgePos | index_staffs_nameAgePos | 74 | NULL | 1 | Using index condition |
+----+-------------+--------+-------+-------------------------+-------------------------+---------+------+------+-----------------------+
1 row in set (0.00 sec)
解决【like ‘%str%’ 】索引失效的问题：使用覆盖索引
创建表
建表 SQL
CREATE TABLE `tbl_user`(
`id` INT(11) NOT NULL AUTO_INCREMENT,
`name` VARCHAR(20) DEFAULT NULL,
`age`INT(11) DEFAULT NULL,
`email` VARCHAR(20) DEFAULT NULL,
PRIMARY KEY(`id`)
)ENGINE=INNODB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
INSERT INTO tbl_user(`name`,`age`,`email`)VALUES('1aa1',21,'a@163.com');
INSERT INTO tbl_user(`name`,`age`,`email`)VALUES('2bb2',23,'b@163.com');
INSERT INTO tbl_user(`name`,`age`,`email`)VALUES('3cc3',24,'c@163.com');
INSERT INTO tbl_user(`name`,`age`,`email`)VALUES('4dd4',26,'d@163.com');
tbl_user 表中的测试数据
mysql> select * from tbl_user;
+----+------+------+-----------+
| id | name | age | email |
+----+------+------+-----------+
| 1 | 1aa1 | 21 | a@163.com |
| 2 | 2bb2 | 23 | b@163.com |
| 3 | 3cc3 | 24 | c@163.com |
| 4 | 4dd4 | 26 | d@163.com |
+----+------+------+-----------+
4 rows in set (0.00 sec)
创建索引
创建索引的 SQL 指令
CREATE INDEX idx_user_nameAge ON tbl_user(name, age);
在 tbl_user 表的 name 字段和 age 字段创建联合索引
mysql> CREATE INDEX idx_user_nameAge ON tbl_user(name, age);
Query OK, 0 rows affected (0.05 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> SHOW INDEX FROM tbl_user;
+----------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| tbl_user | 0 | PRIMARY | 1 | id | A | 4 | NULL | NULL | | BTREE | | |
| tbl_user | 1 | idx_user_nameAge | 1 | name | A | 4 | NULL | NULL | YES | BTREE | | |
| tbl_user | 1 | idx_user_nameAge | 2 | age | A | 4 | NULL | NULL | YES | BTREE | | |
+----------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.00 sec)
测试覆盖索引
如下 SQL 的索引均不会失效：
只要查询的字段能和覆盖索引扯得上关系，并且没有多余字段，覆盖索引就不会失效
但我就想不通了，id 扯得上啥关系。。。
EXPLAIN SELECT name, age FROM tbl_user WHERE NAME LIKE '%aa%';
EXPLAIN SELECT name FROM tbl_user WHERE NAME LIKE '%aa%';
EXPLAIN SELECT age FROM tbl_user WHERE NAME LIKE '%aa%';
EXPLAIN SELECT id FROM tbl_user WHERE NAME LIKE '%aa%';
EXPLAIN SELECT id, name FROM tbl_user WHERE NAME LIKE '%aa%';
EXPLAIN SELECT id, age FROM tbl_user WHERE NAME LIKE '%aa%';
EXPLAIN SELECT id, name, age FROM tbl_user WHERE NAME LIKE '%aa%';
mysql> EXPLAIN SELECT id FROM tbl_user WHERE NAME LIKE '%aa%';
+----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+
| 1 | SIMPLE | tbl_user | index | NULL | idx_user_nameAge | 68 | NULL | 4 | Using where; Using index |
+----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+
1 row in set (0.00 sec)
mysql> EXPLAIN SELECT name, age FROM tbl_user WHERE NAME LIKE '%aa%';
+----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+
| 1 | SIMPLE | tbl_user | index | NULL | idx_user_nameAge | 68 | NULL | 4 | Using where; Using index |
+----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+
1 row in set (0.00 sec)
如下 SQL 的索引均会失效：但凡有多余字段，覆盖索引就会失效
EXPLAIN SELECT * FROM tbl_user WHERE NAME LIKE '%aa%';
EXPLAIN SELECT id, name, age, email FROM tbl_user WHERE NAME LIKE '%aa%';
mysql> EXPLAIN SELECT * FROM tbl_user WHERE NAME LIKE '%aa%';
+----+-------------+----------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | tbl_user | ALL | NULL | NULL | NULL | NULL | 4 | Using where |
+----+-------------+----------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> EXPLAIN SELECT id, name, age, email FROM tbl_user WHERE NAME LIKE '%aa%';
+----+-------------+----------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | tbl_user | ALL | NULL | NULL | NULL | NULL | 4 | Using where |
+----+-------------+----------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
字符串不加单引号索引失效
正常操作，索引没有失效
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
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)
少用or，用它连接时会索引失效
使用 or 连接，会导致索引失效
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 = 'July';
+----+-------------+--------+------+-------------------------+------+---------+------+------+-------------+
| 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)
2、索引优化面试题
索引优化面试题
建表 SQL
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
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
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？
EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' AND c3='a3' AND c4='a4';
即全值匹配
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)
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
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)
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
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)
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
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)
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
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)
EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' ORDER BY c3;
那不就和上面一样的嘛~~~，c4 列都没有用到索引
mysql> EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' 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)
EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' ORDER BY c4;
妈耶，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
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)
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
难道因为排序的时候，c2 紧跟在 c1 之后，所以就不用 filesort 吗？
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)
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
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)
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
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)
EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' AND c5='a5' ORDER BY c2, c3;
和 c5 这个坑爹货没啥关系
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)
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
mysql> EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2='a2' 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 | 62 | const,const | 1 | Using index condition; Using where |
+----+-------------+--------+------+------------------+------------------+---------+-------------+------+------------------------------------+
1 row in set (0.00 sec)
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
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)
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，我只能说是灭绝师太
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)
结论：
- 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
- 定值为常量、范围之后失效，最终看排序的顺序
3、索引失效总结
一般性建议
- 对于单键索引，尽量选择针对当前query过滤性更好的索引
- 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。
- 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
- 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
索引优化的总结
-
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
like SQL 实测
= ‘kk’ ：key_len = 93 ，请记住此参数的值，后面有用
----+-------------+--------+------+------------------+------------------+---------+-------------------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+------------------+------------------+---------+-------------------+------+-----------------------+
| 1 | SIMPLE | test03 | ref | idx_test03_c1234 | idx_test03_c1234 | 93 | const,const,const | 1 | Using index condition |
+----+-------------+--------+------+------------------+------------------+---------+-------------------+------+-----------------------+
1 row in set (0.00 sec)
like ‘kk%’：
key_len = 93 ，和上面一样，说明 c1 c2 c3 都用到了索引
type = range 表明这是一个范围搜索
mysql> EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2 like 'kk%' 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 | 93 | NULL | 1 | Using index condition |
+----+-------------+--------+-------+------------------+------------------+---------+------+------+-----------------------+
1 row in set (0.00 sec)
like ‘%kk’ 和 like ‘%kk%’ ：key_len = 31 ，表示只有 c1 用到了索引
mysql> EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2 like '%kk' AND c3='a3';
+----+-------------+--------+------+------------------+------------------+---------+-------+------+-----------------------+
| 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 |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+-----------------------+
1 row in set (0.00 sec)
mysql> EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2 like '%kk%' AND c3='a3';
+----+-------------+--------+------+------------------+------------------+---------+-------+------+-----------------------+
| 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 |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+-----------------------+
1 row in set (0.00 sec)
like ‘k%kk%’ ：key_len = 93 ，表示 c1 c2 c3 都用到了索引
mysql> EXPLAIN SELECT * FROM test03 WHERE c1='a1' AND c2 like 'k%kk%' 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 | 93 | NULL | 1 | Using index condition |
+----+-------------+--------+-------+------------------+------------------+---------+------+------+-----------------------+
1 row in set (0.00 sec)
索引优化的总结
全值匹配我最爱， 最左前缀要遵守；
带头大哥不能死， 中间兄弟不能断；
索引列上少计算， 范围之后全失效；
LIKE 百分写最右，覆盖索引不写 *；
不等空值还有OR，索引影响要注意；
VAR 引号不可丢， SQL 优化有诀窍。
