1、單表索引優(yōu)化
單表索引優(yōu)化分析
創(chuàng)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');
表中的測(cè)試數(shù)據(jù)
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)
查詢案例
查詢category_id為1且comments 大于1的情況下娱仔，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)
此時(shí) article 表中只有一個(gè)主鍵索引
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 語(yǔ)句的執(zhí)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)
結(jié)論：
- 很顯然霉涨，type是ALL颖榜，即最壞的情況叶摄。
- Extra 里還出現(xiàn)了Using filesort，也是最壞的情況碗淌。
- 優(yōu)化是必須的盏求。
開(kāi)始優(yōu)化：新建索引
創(chuàng)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 列上建立聯(lián)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)
再次執(zhí)行查詢：type變成了range亿眠，這是可以忍受的碎罚。但是extra里使用Using filesort仍是無(wú)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
- 但是我們已經(jīng)建立了索引纳像，為啥沒(méi)用呢荆烈？
- 這是因?yàn)榘凑誃+Tree索引的工作原理阿纤，先排序 category_id乍迄，如果遇到相同的 category_id 則再排序comments贼邓，如果遇到相同的 comments 則再排序 views鄙陡。
- 當(dāng)comments字段在聯(lián)合索引里處于中間位置時(shí)，因?yàn)閏omments>1條件是一個(gè)范圍值（所謂 range）管挟，MySQL 無(wú)法利用索引再對(duì)后面的views部分進(jìn)行檢索缓熟，即 range 類(lèi)型查詢字段后面的索引無(wú)效移稳。
將查詢條件中的 comments > 1 改為 comments = 1 犀勒，發(fā)現(xiàn) Use filesort 神奇地消失了屎飘，從這點(diǎn)可以驗(yàn)證：范圍后的索引會(huì)導(dǎo)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)
刪除索引
刪除索引的 SQL 指令
DROP INDEX idx_article_ccv ON article;
刪除剛才創(chuàng)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)
再次創(chuàng)建索引
創(chuàng)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）的索引會(huì)失效，這次我們建立索引時(shí)贾费，直接拋棄 comments 列钦购，先利用 category_id 和 views 的聯(lián)合索引查詢所需要的數(shù)據(jù)，再?gòu)钠渲腥〕?comments > 1 的數(shù)據(jù)（我覺(jué)著應(yīng)該是這樣的）
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)
再次執(zhí)行查詢：可以看到褂萧，type變?yōu)榱藃ef押桃，Extra中的Using filesort也消失了，結(jié)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)
為了不影響之后的測(cè)試箱玷，刪除該表的 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怨规、兩表索引優(yōu)化
兩表索引優(yōu)化分析：主外鍵
創(chuàng)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 表中的測(cè)試數(shù)據(jù)
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 表中的測(cè)試數(shù)據(jù)
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)
查詢案例
實(shí)現(xiàn)兩表的連接陌宿，連接條件是 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 語(yǔ)句的性能锡足，可以看到：驅(qū)動(dòng)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)
結(jié)論：
- type 有 All 波丰，rows 為表中數(shù)據(jù)總行數(shù)，說(shuō)明 class 和 book 進(jìn)行了全表檢索
- 即每次 class 表對(duì) book 表進(jìn)行左外連接時(shí)舶得，都需要在 book 表中進(jìn)行一次全表檢索
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)
測(cè)試結(jié)果：可以看到第二行的type變?yōu)榱藃ef掰烟，rows也變成了優(yōu)化比較明顯。
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：
- 這是由左連接特性決定的沐批。LEFT JOIN條件用于確定如何從右表搜索行纫骑，左邊一定都有，所以右邊是我們的關(guān)鍵點(diǎn)九孩，一定需要建立索引先馆。
- 左表連接右表，則需要拿著左表的數(shù)據(jù)去右表里面查躺彬，索引需要在右表中建立索引
將左連接修改為右連接
刪除之前 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);
再次執(zhí)行左連接煤墙，涼涼~~~
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)
別怕，我們來(lái)執(zhí)行右連接：可以看到第二行的type變?yōu)榱藃ef宪拥，rows也變成了優(yōu)化比較明顯仿野。
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：
- 這是因?yàn)?strong>RIGHT JOIN條件用于確定如何從左表搜索行，右邊一定都有她君，所以左邊是我們的關(guān)鍵點(diǎn)脚作，一定需要建立索引。
- class RIGHT JOIN book ：book 里面的數(shù)據(jù)一定存在于結(jié)果集中缔刹，我們需要拿著book 表中的數(shù)據(jù)球涛，去 class 表中搜索，所以索引需要建立在 class 表中
3校镐、三表索引優(yōu)化
三表索引優(yōu)化分析
創(chuàng)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 表中的測(cè)試數(shù)據(jù)
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)
查詢案例
實(shí)現(xiàn)三表的連接查詢：
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)
結(jié)論：
- type 有All 宾符，rows 為表數(shù)據(jù)總行數(shù)，說(shuō)明 class灭翔、 book 和 phone 表都進(jìn)行了全表檢索
- Extra 中 Using join buffer 魏烫，表明連接過(guò)程中使用了 join 緩沖區(qū)
創(chuàng)建索引
創(chuàng)建索引的 SQL 語(yǔ)句
ALTER TABLE book ADD INDEX Y (card);
ALTER TABLE phone ADD INDEX Z (card);
進(jìn)行 LEFT JOIN ，永遠(yuǎn)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)
執(zhí)行查詢：后2行的type都是ref肝箱，且總rows優(yōu)化很好哄褒，效果不錯(cuò)。因此索引最好設(shè)置在需要經(jīng)常查詢的字段中煌张。
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 語(yǔ)句優(yōu)化的結(jié)論
將 left join 看作是兩層嵌套 for 循環(huán)
1呐赡、盡可能減少Join語(yǔ)句中的NestedLoop的循環(huán)總次數(shù)；
2骏融、永遠(yuǎn)用小結(jié)果集驅(qū)動(dòng)大的結(jié)果集（在大結(jié)果集中建立索引链嘀，在小結(jié)果集中遍歷全表）萌狂；
3、優(yōu)先優(yōu)化NestedLoop的內(nèi)層循環(huán)怀泊；
4茫藏、保證Join語(yǔ)句中被驅(qū)動(dòng)表上Join條件字段已經(jīng)被索引；
5霹琼、當(dāng)無(wú)法保證被驅(qū)動(dòng)表的Join條件字段被索引且內(nèi)存資源充足的前提下务傲，不要太吝惜JoinBuffer的設(shè)置；
我的理解
1枣申、使用小表驅(qū)動(dòng)大表售葡，這就相當(dāng)于外層 for 循環(huán)的次數(shù)少，內(nèi)層 for 循環(huán)的次數(shù)多
2忠藤、然后我們?cè)诖蟊碇薪⒘怂饕铮@樣內(nèi)層 for 循環(huán)的效率明顯提高
3、綜上模孩，使用小表驅(qū)動(dòng)大表尖阔，在大表中建立了索引
索引失效
索引失效（應(yīng)該避免）
創(chuàng)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'年齡',
`pos` VARCHAR(20) NOT NULL DEFAULT'' COMMENT'職位',
`add_time` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT'入職時(shí)間'
)CHARSET utf8 COMMENT'員工記錄表';
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 表中的測(cè)試數(shù)據(jù)
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 表中的復(fù)合索引：name、age瓜贾、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诺祸、索引失效準(zhǔn)則
1、全值匹配我最?lèi)?ài)
2祭芦、最佳左前綴法則：如果索引了多例筷笨，要遵守最左前綴法則。指的是查詢從索引的最左前列開(kāi)始并且不跳過(guò)索引中的列龟劲。
3胃夏、不在索引列上做任何操作（計(jì)算、函數(shù)昌跌、（自動(dòng)or手動(dòng)）類(lèi)型轉(zhuǎn)換）仰禀，會(huì)導(dǎo)致索引失效而轉(zhuǎn)向全表掃描
4、存儲(chǔ)引擎不能使用索引中范圍條件右邊的列
5蚕愤、盡量使用覆蓋索引（只訪問(wèn)索引的查詢（索引列和查詢列一致））答恶，減少select *
6、mysql在使用不等于（!=或者<>）的時(shí)候無(wú)法使用索引會(huì)導(dǎo)致全表掃描
7萍诱、is null悬嗓，is not null 也無(wú)法使用索引（早期版本不能走索引，后續(xù)版本應(yīng)該優(yōu)化過(guò)裕坊，可以走索引）
8包竹、like以通配符開(kāi)頭（’%abc…’）mysql索引失效會(huì)變成全表掃描操作
9、字符串不加單引號(hào)索引失效
10、少用or周瞎，用它連接時(shí)會(huì)索引失效
最佳左匹配法則：帶頭大哥不能死苗缩，中間兄弟不能斷
只有帶頭大哥 name 時(shí)
key = index_staffs_nameAgePos 表明索引生效
ref = const ：這個(gè)常量就是查詢時(shí)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)
帶頭大哥 name 帶上小弟 age
key = index_staffs_nameAgePos 表明索引生效
ref = const,const：兩個(gè)常量分別為 ‘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)
帶頭大哥 name 帶上小弟 age ，小弟 age 帶上小小弟 pos
key = index_staffs_nameAgePos 表明索引生效
ref = const,const,const ：三個(gè)常量分別為 ‘July’声诸、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)
帶頭大哥 name 掛了
key = NULL 說(shuō)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)
帶頭大哥 name 沒(méi)掛酱讶，小弟 age 跑了
key = index_staffs_nameAgePos 說(shuō)明索引沒(méi)有失效
ref = const 表明只使用了一個(gè)常量，即第二個(gè)常量（pos = ‘dev’）沒(méi)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)
在索引列上進(jìn)行計(jì)算双絮，會(huì)導(dǎo)致索引失效浴麻，進(jìn)而轉(zhuǎn)向全表掃描
不對(duì)帶頭大哥 name 進(jìn)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)
對(duì)帶頭大哥 name 進(jìn)行操作：使用 LEFT 函數(shù)截取子串
key = NULL 表明索引生效
type = ALL 表明進(jìn)行了全表掃描
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)
范圍之后全失效
精確匹配
type = ref 表示非唯一索引掃描得问，SQL 語(yǔ)句將返回匹配某個(gè)單獨(dú)值的所有行囤攀。
key_len = 140 表明表示索引中使用的字節(jié)數(shù)
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)
將 age 改為范圍匹配
type = range 表示范圍掃描
key = index_staffs_nameAgePos 表示索引并沒(méi)有失效
key_len = 78 ，ref = NULL 均表明范圍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)
盡量使用覆蓋索引（只訪問(wèn)索引的查詢（索引列和查詢列一致））宫纬，減少 select *
SELECT * 的寫(xiě)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)
覆蓋索引的寫(xiě)法：Extra = Using where; Using index 焚挠，Using index 表示使用索引列進(jìn)行查詢，將大大提高查詢的效率
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)
覆蓋索引中包含 range 條件：type = ref 并且 Extra = Using where; Using index 漓骚，雖然在查詢條件中使用了 范圍搜索蝌衔，但是由于我們只需要查找索引列，所以無(wú)需進(jìn)行全表掃描
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<>）的時(shí)候無(wú)法使用索引會(huì)導(dǎo)致全表掃描
在使用 != 會(huì) <> 時(shí)會(huì)導(dǎo)致索引失效：
key = null 表示索引失效
rows = 3 表示進(jìn)行了全表掃描
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蝌蹂，is not null 也無(wú)法使用索引
is null噩斟，is not null 會(huì)導(dǎo)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 % 寫(xiě)應(yīng)該最右邊，在左邊容易導(dǎo)致索引失效
staffs 表的索引關(guān)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 % 寫(xiě)在左邊的情況
type = All 孤个，rows = 3 表示進(jìn)行了全表掃描
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 % 寫(xiě)在右邊的情況：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)
解決【like ‘%str%’ 】索引失效的問(wèn)題：使用覆蓋索引
創(chuàng)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 表中的測(cè)試數(shù)據(jù)
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)
創(chuàng)建索引
創(chuàng)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 字段創(chuàng)建聯(lián)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)
測(cè)試覆蓋索引
如下 SQL 的索引均不會(huì)失效：
只要查詢的字段能和覆蓋索引扯得上關(guān)系剃允，并且沒(méi)有多余字段，覆蓋索引就不會(huì)失效
但我就想不通了齐鲤，id 扯得上啥關(guān)系斥废。。给郊。
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 的索引均會(huì)失效：但凡有多余字段牡肉，覆蓋索引就會(huì)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)
字符串不加單引號(hào)索引失效
正常操作，索引沒(méi)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)
如果字符串忘記寫(xiě) ' ' 淆九，那么 mysql 會(huì)為我們進(jìn)行隱式的類(lèi)型轉(zhuǎn)換统锤，但凡進(jìn)行了類(lèi)型轉(zhuǎn)換，索引都會(huì)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炭庙，用它連接時(shí)會(huì)索引失效
使用 or 連接饲窿，會(huì)導(dǎo)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、索引優(yōu)化面試題
索引優(yōu)化面試題
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 表中的測(cè)試數(shù)據(jù)
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)
問(wèn)題：我們創(chuàng)建了復(fù)合索引idx_test03_c1234煤搜，根據(jù)以下SQL分析下索引使用情況免绿？
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 優(yōu)化器進(jìn)行了優(yōu)化，所以我們的索引都生效了
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 列使用了索引進(jìn)行排序擦盾，并沒(méi)有進(jìn)行查找嘲驾，導(dǎo)致 c4 無(wú)法用索引進(jìn)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 優(yōu)化器進(jìn)行了優(yōu)化淌哟，所以我們的索引都生效了，在 c4 時(shí)進(jìn)行了范圍搜索
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 列將索引用于排序辽故，而不是查找徒仓，c4 列沒(méi)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;
那不就和上面一樣的嘛~~~，c4 列都沒(méi)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;
媽耶誊垢，因?yàn)樗饕⒌捻樞蚝褪褂玫捻樞虿灰恢碌舫冢瑢?dǎo)致 mysql 動(dòng)用了文件排序
看到 Using filesort 就要知道：此句 SQL 必須優(yōu)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 一個(gè)字段索引，但是c2喂走、c3用于排序殃饿，無(wú)filesort
難道因?yàn)榕判虻臅r(shí)候，c2 緊跟在 c1 之后芋肠，所以就不用 filesort 嗎乎芳？
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;
出現(xiàn)了filesort，我們建的索引是1234帖池，它沒(méi)有按照順序來(lái)奈惑，32顛倒了
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、c2兩個(gè)字段索引睡汹，但是c2肴甸、c3用于排序，無(wú)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 這個(gè)坑爹貨沒(méi)啥關(guān)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;
注意查詢條件 c2=‘a(chǎn)2’ 囚巴，我都把 c2 查出來(lái)了（c2 為常量）原在，我還給它排序作甚，所以沒(méi)有產(chǎn)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;
順序?yàn)?1 2 3 文兢，沒(méi)有產(chǎn)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 表面上叫分組晤斩，分組之前必排序，group by 和 order by 在索引上的問(wèn)題基本是一樣的
Using temporary; Using filesort 兩個(gè)都有姆坚，我只能說(shuō)是滅絕師太
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)
結(jié)論：
- group by 基本上都需要進(jìn)行排序澳泵，但凡使用不當(dāng)，會(huì)有臨時(shí)表產(chǎn)生
- 定值為常量兼呵、范圍之后失效兔辅，最終看排序的順序
3、索引失效總結(jié)
一般性建議
- 對(duì)于單鍵索引击喂，盡量選擇針對(duì)當(dāng)前query過(guò)濾性更好的索引
- 在選擇組合索引的時(shí)候维苔，當(dāng)前query中過(guò)濾性最好的字段在索引字段順序中，位置越靠左越好懂昂。
- 在選擇組合索引的時(shí)候介时，盡量選擇可以能包含當(dāng)前query中的where子句中更多字段的索引
- 盡可能通過(guò)分析統(tǒng)計(jì)信息和調(diào)整query的寫(xiě)法來(lái)達(dá)到選擇合適索引的目的
索引優(yōu)化的總結(jié)
-
like 后面以常量開(kāi)頭，比如 like ‘kk%’ 和 like ‘k%kk%’ ，可以理解為就是常量
like SQL 實(shí)測(cè)
= ‘kk’ ：key_len = 93 沸柔，請(qǐng)記住此參數(shù)的值循衰，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 ，和上面一樣褐澎，說(shuō)明 c1 c2 c3 都用到了索引
type = range 表明這是一個(gè)范圍搜索
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 会钝，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)
索引優(yōu)化的總結(jié)
全值匹配我最?lèi)?ài)工三， 最左前綴要遵守迁酸；
帶頭大哥不能死， 中間兄弟不能斷俭正；
索引列上少計(jì)算奸鬓， 范圍之后全失效；
LIKE 百分寫(xiě)最右段审，覆蓋索引不寫(xiě) *全蝶；
不等空值還有OR闹蒜，索引影響要注意寺枉；
VAR 引號(hào)不可丟， SQL 優(yōu)化有訣竅绷落。
