一、字段基本解讀

+-----+-------------+-------+------+---------------+------------+---------------+-----------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +-----+-------------+-------+------+---------------+------------+---------------+-----------+------+-------+ | 編號 | 查詢類型 | 表 | 類型 | 預測用到的索引 | 實際使用索引 | 實際使用索引長度 | 表之間引用 | 行數 | 額外信息 | +-----+-------------+-------+------+---------------+------------+---------------+-----------+------+-------+

一般來說關注type,key,key_len,Extra段進行SQL優化即可。

二、創建例表

#創建用戶表 CREATE TABLE `users`(`id` INT UNSIGNED AUTO_INCREMENT,`name` VARCHAR(8) NOT NULL DEFAULT '',`age` TINYINT NOT NULL DEFAULT 0,`email` VARCHAR(32),`info` VARCHAR(255) COMMENT '無索引',PRIMARY KEY(id) COMMENT '主鍵索引',UNIQUE INDEX ui_email(email) COMMENT '唯一索引',INDEX i_age(age) COMMENT '普通索引',INDEX i_name_age(name,age) COMMENT '復合索引' )ENGINE = INNODB DEFAULT CHARSET = utf8; #插入一條 INSERT INTO `users`(`name`,`age`,`email`,`info`) VALUES('voyager',24,'772532526@qq.com','人生若只如初見'); #創建卡片表，用戶與卡片一對多 CREATE TABLE `card`(`cid` INT UNSIGNED AUTO_INCREMENT,`uid` INT NOT NULL DEFAULT 0,`card` VARCHAR(255) NOT NULL DEFAULT '',PRIMARY KEY(`cid`)#暫不為uid加索引 )ENGINE = InnoDB DEFAULT CHARSET = UTF8; #插入兩條數據 INSERT INTO `card`(`uid`,`card`) VALUES(1,'何事秋風悲畫扇'),(1,'等閑變卻故人心，卻道故人心易變');

三、單獨釋義

1. id值

標記分解的sql執行順序，非直觀的從上至下的順序。
id值相同，從上至下執行。
id值不同，從大到小執行。

以此結論，反觀實例，僅觀察 id 和 table 部分。

#SQL1 mysql> EXPLAIN SELECT * FROM users,card WHERE id = uid AND cid = 1; +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+ | 1 | SIMPLE | card | const | PRIMARY | PRIMARY | 4 | const | 1 | | | 1 | SIMPLE | users | const | PRIMARY | PRIMARY | 4 | const | 1 | | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+

SQL1，雖然以cid為條件置于后邊，但因其為主鍵，SQL解釋器知道使用它先篩選更快。

#SQL2 mysql> EXPLAIN SELECT * FROM users,card WHERE uid = 1 AND id = uid ; +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+ | 1 | SIMPLE | users | const | PRIMARY | PRIMARY | 4 | const | 1 | | | 1 | SIMPLE | card | ALL | NULL | NULL | NULL | NULL | 2 | Using where | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+

SQL2，uid作為條件，卻先篩選users表，原因為：作為條件的字段非索引，則先篩選數據量小的表。

#SQL3 mysql> EXPLAIN SELECT * FROM users WHERE id = (SELECT uid FROM card WHERE cid = 2); +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+ | 1 | PRIMARY | users | const | PRIMARY | PRIMARY | 4 | const | 1 | | | 2 | SUBQUERY | card | const | PRIMARY | PRIMARY | 4 | const | 1 | | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+

SQL3，加入一個子查詢，id不同，先進行2子查詢，再進行1主查詢。


2.select_type查詢類型

類型出現情況

SIMPLE	簡單查詢，不包含子查詢、UNION，例SQL1
PRIMARY	復雜查詢中的主查詢，例SQL3
UBQUERY	包含子查詢語句的子查詢可能出現，例SQL3
UNION	聯合查詢，t1 UNION t2 中的t2
UNION RESULT	聯合結果
DERIVED	衍生查詢，一般為產生的臨時表

舉例一個UNION的

#SQL4 mysql> EXPLAIN SELECT uid FROM card WHERE cid = 1 UNION SELECT uid FROM card WHERE cid = 2; +------+--------------+------------+-------+---------------+---------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+--------------+------------+-------+---------------+---------+---------+-------+------+-------+ | 1 | PRIMARY | card | const | PRIMARY | PRIMARY | 4 | const | 1 | | | 2 | UNION | card | const | PRIMARY | PRIMARY | 4 | const | 1 | | | NULL | UNION RESULT | <union1,2> | ALL | NULL | NULL | NULL | NULL | NULL | | +------+--------------+------------+-------+---------------+---------+---------+-------+------+-------+

還有一些其他的，不詳細列出。


3.table表
一般指查詢的表，對于帶尖括號的，表示select_type + id的指向。例如SQL4，<union1,2>表示id為1和2聯合出來的表。

4.type索引類型
速度null > syetem > const > eq_ref > ref > range > index > all

類型出現情況

null	甚至不需要訪問索引表，例如主鍵作為條件超過當前表主鍵最大值;
system	const的特殊情況，只有一條數據的系統表，或衍生表只有一條數據的主查詢？？
const	使用唯一索引等價查詢，僅能匹配到一條數據
eq_ref	使用唯一索引作為關聯條件，匹配多條不重復數據
ref	普通索引等價
range	檢索給定范圍的索引 ， > 、< 、>= 、<=、between and
index	僅查詢索引表
all	遍歷全表以找到匹配的行

例子在后邊。


5.posibble_keys預測用到的索引
寫sql語句時，有些看起來會使用的索引，但實際可能拋棄索引，遍歷全表。

6.keys實際使用的索引

7.key_lengths實際使用的索引長度

一般用于判斷復合索引是否被完全使用。
utf8中一個字符占3個字節。
null占1個字節。
可變長度占2個字節。

#SQL5 mysql> EXPLAIN SELECT * FROM users WHERE name = 'a' AND age = 2; +----+-------------+-------+------+------------------+-------+---------+-------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+------------------+-------+---------+-------+------+-------------+ | 1 | SIMPLE | users | ref | i_age,i_name_age | i_age | 1 | const | 1 | Using where | +----+-------------+-------+------+------------------+-------+---------+-------+------+-------------+

SQL5，我本意是想用到聯合索引i_name_age，但是實際上被索引i_age干擾了，這也解釋了possible_keys和keys的不同，而i_age作為tinyint占1個字符，所以key_len值為1。好的，這種時候需要考慮i_age的必要性，要么刪掉此索引，要么強制使用索引。
這里我選擇了先刪除在添加回來，卻發現了一個有意思的事情。

mysql> ALTER TABLE users DROP INDEX i_age; #SQL6 mysql> EXPLAIN SELECT * FROM users WHERE name = 'a' AND age = 2; +----+-------------+-------+-------+---------------+------------+---------+------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+------------+---------+------+------+-----------------------+ | 1 | SIMPLE | users | range | i_name_age | i_name_age | 27 | NULL | 1 | Using index condition | +----+-------------+-------+-------+---------------+------------+---------+------+------+-----------------------+

沒問題，name為varchar(8) not null，8 * 3 + 2 + 1 = 27。
然后為了測試把i_age加回來。

mysql> ALTER TABLE users ADD INDEX i_age(`age`); #SQL7 mysql> EXPLAIN SELECT * FROM users WHERE name = 'a' AND age = 2; +----+-------------+-------+------+------------------+------------+---------+-------------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+------------------+------------+---------+-------------+------+-----------------------+ | 1 | SIMPLE | users | ref | i_name_age,i_age | i_name_age | 27 | const,const | 1 | Using index condition | +----+-------------+-------+------+------------------+------------+---------+-------------+------+-----------------------+

哎？發現這回i_age并沒有干擾到復合索引的使用。我猜測與索引建立的順序有關，觀察兩次possible_keys索引順序，并刪除再添加回來i_name_age情況又和SQL5一樣，也驗證了這一點。

注意復合索引與普通索引的關系及創建順序。

8.ref
表示上述表的連接匹配條件，即哪些列或常量被用于查找索引列上的值

9.rows
根據表統計信息及索引選用情況，估算的找到所需的記錄所需要讀取的行數

10.extra

名詞解釋

Using index	性能提升，索引覆蓋，此查詢僅查詢索引不需要回表查詢
Using where	在查找使用索引的情況下，需要回表去查詢所需的數據
Using filesort	性能消耗大，需要額外一次文件排序
Using temporary	性能消耗大，用到臨時表，常見于排序和分組查詢
Using join buffer	連接緩存
Using where; Using index	查找使用了索引，但是需要的數據都在索引列中能找到，所以不需要回表查詢數據

實例

為了更易懂，添加一列money，刪除索引i_name_age和i_age，創建i_money_name_age。

ALTER TABLE users ADD COLUMN money INT NOT NULL DEFAULT 10; ALTER TABLE DROP INDEX i_age; ALTER TABLE DROP INDEX i_name_age; ALTER TABLE users ADD INDEX i_money_name_age(money,name,age);

復合索引使用必須從左開始，且不能跨列，不能計算。#SQL8 mysql> EXPLAIN SELECT * FROM users WHERE age = 12 AND name = '12' AND money = 12; +----+-------------+-------+------+------------------+------------------+---------+-------------------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+------------------+------------------+---------+-------------------+------+-----------------------+ | 1 | SIMPLE | users | ref | i_money_name_age | i_money_name_age | 31 | const,const,const | 1 | Using index condition | +----+-------------+-------+------+------------------+------------------+---------+-------------------+------+-----------------------+ SQL8沒按順序但使用了復合索引，是因為SQL優化器的緣故把條件重排序了。#SQL9 mysql> EXPLAIN SELECT * FROM users WHERE name = '12' AND age = 12; +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | users | ALL | NULL | NULL | NULL | NULL | 2 | Using where | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ SQL9沒從左開始，都未使用上索引。#SQL10 mysql> EXPLAIN SELECT * FROM users WHERE money = 12 AND age = 12; +----+-------------+-------+------+------------------+------------------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+------------------+------------------+---------+-------+------+-----------------------+ | 1 | SIMPLE | users | ref | i_money_name_age | i_money_name_age | 4 | const | 1 | Using index condition | +----+-------------+-------+------+------------------+------------------+---------+-------+------+-----------------------+ SQL10，觀察索引長度，因為跨過name，所以age沒有使用上索引。

沒有where時，注意order by。#SQL11 mysql> EXPLAIN SELECT * FROM users ORDER BY money; +----+-------------+-------+------+---------------+------+---------+------+------+----------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+----------------+ | 1 | SIMPLE | users | ALL | NULL | NULL | NULL | NULL | 2 | Using filesort | +----+-------------+-------+------+---------------+------+---------+------+------+----------------+ SQL11，此時select * 導致order by不使用索引。#SQL12 mysql> EXPLAIN SELECT id,money,age FROM users ORDER BY money; +----+-------------+-------+-------+---------------+------------------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+------------------+---------+------+------+-------------+ | 1 | SIMPLE | users | index | NULL | i_money_name_age | 31 | NULL | 2 | Using index | +----+-------------+-------+-------+---------------+------------------+---------+------+------+-------------+ SQL12，如果選中復合索引里包含的字段和主鍵，會使用索引。

注意索引建立的排序方式#SQL13 mysql> EXPLAIN SELECT id,money,age FROM users ORDER BY money,name,age DESC; +----+-------------+-------+-------+---------------+------------------+---------+------+------+-----------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+------------------+---------+------+------+-----------------------------+ | 1 | SIMPLE | users | index | NULL | i_money_name_age | 31 | NULL | 2 | Using index; Using filesort | +----+-------------+-------+-------+---------------+------------------+---------+------+------+-----------------------------+ SQL13，這里產生了Using filesort，因為默認索引建立順序都是ASC，三個字段順序必須一致。

關于order by使用索引

在這里插入圖片描述

疑惑

#SQL9 mysql> EXPLAIN SELECT * FROM users WHERE name = 'voyager' AND age > 12; +----+-------------+-------+-------+-----------------------------+------------+---------+------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+-----------------------------+------------+---------+------+------+-----------------------+ | 1 | SIMPLE | users | range | i_name_age,i_age,i_age_name | i_name_age | 27 | NULL | 2 | Using index condition | +----+-------------+-------+-------+-----------------------------+------------+---------+------+------+-----------------------+ 1 row in setmysql> EXPLAIN SELECT name,age FROM users WHERE name = 'voyager' AND age > 12; +----+-------------+-------+------+-----------------------------+------------+---------+-------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+-----------------------------+------------+---------+-------+------+--------------------------+ | 1 | SIMPLE | users | ref | i_name_age,i_age,i_age_name | i_name_age | 26 | const | 2 | Using where; Using index | +----+-------------+-------+------+-----------------------------+------------+---------+-------+------+--------------------------+ 1 row in setmysql> EXPLAIN SELECT name,age FROM users WHERE name = 'voyager' ORDER BY age; +----+-------------+-------+------+---------------+------------+---------+-------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------------+---------+-------+------+--------------------------+ | 1 | SIMPLE | users | ref | i_name_age | i_name_age | 26 | const | 2 | Using where; Using index | +----+-------------+-------+------+---------------+------------+---------+-------+------+--------------------------+

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