文章目錄

• 生猛干貨
• DB Version
• Table
• Case 1 : 聯(lián)合索引第一個字段用范圍不一定會走索引
• • 優(yōu)化一 強制走索引 force index(idx_name_age_position)
  • 優(yōu)化二 覆蓋索引優(yōu)化
• Case 2 : in和or在表數(shù)據(jù)量比較大的情況會走索引，在表記錄不多的情況下會選擇全表掃描
• Case 3 : like KK% 一般情況都會走索引
• • 特殊例子
• 搞定MySQL

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生猛干貨

帶你搞定MySQL實戰(zhàn)，輕松對應海量業(yè)務處理及高并發(fā)需求，從容應對大場面試

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DB Version

mysql> select version(); +------------+ | version() | +------------+ | 5.7.29-log | +------------+ 1 row in set

默認隔離級別 RR 可重復讀

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Table

CREATE TABLE `employees` (`id` int(11) NOT NULL AUTO_INCREMENT,`name` varchar(24) NOT NULL DEFAULT '' COMMENT '姓名',`age` int(11) NOT NULL DEFAULT '0' COMMENT '年齡',`position` varchar(20) NOT NULL DEFAULT '' COMMENT '職位',`hire_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '入職時間',PRIMARY KEY (`id`),KEY `idx_name_age_position` (`name`,`age`,`position`) USING BTREE ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8 COMMENT='員工記錄表';

重點

主鍵id PRIMARY KEY (id)

聯(lián)合索引 KEY idx_name_age_position (name,age,position) USING BTREE

我們向表里寫入10萬來條數(shù)據(jù)

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Case 1 : 聯(lián)合索引第一個字段用范圍不一定會走索引

mysql> EXPLAIN SELECT * FROM employees WHERE name > 'LiLei' AND age = 22 AND position ='manager'; +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+ | 1 | SIMPLE | employees | NULL | range | idx_name_age_position | idx_name_age_position | 74 | NULL | 1 | 5 | Using index condition | +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+ 1 row in setmysql> EXPLAIN SELECT * FROM employees WHERE name > 'Artisan' AND age = 22 AND position ='manager'; +----+-------------+-----------+------------+------+-----------------------+------+---------+------+--------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------------+------+-----------------------+------+---------+------+--------+----------+-------------+ | 1 | SIMPLE | employees | NULL | ALL | idx_name_age_position | NULL | NULL | NULL | 100175 | 0.5 | Using where | +----+-------------+-----------+------------+------+-----------------------+------+---------+------+--------+----------+-------------+ 1 row in setmysql>

當然了，也不是所有的情況都不走索引， MySQL會基于Cost選擇一個合適的 ，如果沒有走索引，可能mysql內部可能覺得第一個字段就用范圍，結果集應該很大，回表效率不高，還不如就全表掃描

如果沒有走索引想要去優(yōu)化的話怎么辦呢？

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優(yōu)化一 強制走索引 force index(idx_name_age_position)

mysql> EXPLAIN SELECT * FROM employees force index(idx_name_age_position) WHERE name > 'Artisan' AND age = 22 AND position ='manager'; +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+-------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+-------+----------+-----------------------+ | 1 | SIMPLE | employees | NULL | range | idx_name_age_position | idx_name_age_position | 74 | NULL | 50087 | 1 | Using index condition | +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+-------+----------+-----------------------+ 1 row in set

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優(yōu)化二 覆蓋索引優(yōu)化

mysql> EXPLAIN SELECT name , age , position FROM employees WHERE name > 'Artisan' AND age = 22 AND position ='manager'; +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+-------+----------+--------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+-------+----------+--------------------------+ | 1 | SIMPLE | employees | NULL | range | idx_name_age_position | idx_name_age_position | 74 | NULL | 50087 | 1 | Using where; Using index | +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+-------+----------+--------------------------+ 1 row in setmysql>

name , age , position 是聯(lián)合索引，在索引樹上，同時索引樹上的葉子節(jié)點還會關聯(lián)一個主鍵id , 如果查詢 * 的話，還要根據(jù)id去主鍵索引上去查找其他字段，需要回表， 如果僅查詢二級索引樹idx_name_age_position上的字段，那就無需回表操作了，效率自然高一些。

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Case 2 : in和or在表數(shù)據(jù)量比較大的情況會走索引，在表記錄不多的情況下會選擇全表掃描

mysql> EXPLAIN SELECT * FROM employees WHERE name in ('LiLei','HanMeimei','Lucy') AND age = 22 AND position ='manager'; +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+ | 1 | SIMPLE | employees | NULL | range | idx_name_age_position | idx_name_age_position | 140 | NULL | 3 | 100 | Using index condition | +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+ 1 row in setmysql> EXPLAIN SELECT * FROM employees WHERE (name = 'LiLei' or name = 'HanMeimei') AND age = 22 AND position ='manager'; +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+ | 1 | SIMPLE | employees | NULL | range | idx_name_age_position | idx_name_age_position | 140 | NULL | 2 | 100 | Using index condition | +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+ 1 row in set

再搞個小表 ，和 employees 一模一樣哈，連索引也得一樣，插入3條數(shù)據(jù) 。

CREATE TABLE `employees_2` (`id` int(11) NOT NULL AUTO_INCREMENT,`name` varchar(24) NOT NULL DEFAULT '' COMMENT '姓名',`age` int(11) NOT NULL DEFAULT '0' COMMENT '年齡',`position` varchar(20) NOT NULL DEFAULT '' COMMENT '職位',`hire_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '入職時間',PRIMARY KEY (`id`),KEY `idx_name_age_position` (`name`,`age`,`position`) USING BTREE ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8 COMMENT='員工記錄表';INSERT INTO employees_2(name,age,position,hire_time) VALUES('LiLei',22,'manager',NOW()); INSERT INTO employees_2(name,age,position,hire_time) VALUES('HanMeimei', 23,'dev',NOW()); INSERT INTO employees_2(name,age,position,hire_time) VALUES('Lucy',23,'dev',NOW());

mysql> EXPLAIN SELECT * FROM employees_2 WHERE name in ('LiLei','HanMeimei','Lucy') AND age = 22 AND position ='manager'; +----+-------------+-------------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | employees_2 | NULL | ALL | idx_name_age_position | NULL | NULL | NULL | 3 | 100 | Using where | +----+-------------+-------------+------------+------+-----------------------+------+---------+------+------+----------+-------------+ 1 row in setmysql>

為什么呢？ 就幾條數(shù)據(jù)的話， 結合B+樹的結構， MySQL認為從根節(jié)點開始向下找，還不如直接從葉子節(jié)點從頭開始掃描快呢

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Case 3 : like KK% 一般情況都會走索引

結合索引樹 ， like KK% 可以理解為就是按照 = KK 查詢

mysql> EXPLAIN SELECT * FROM employees WHERE name like 'LiLei%' AND age = 22 AND position ='manager'; +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+ | 1 | SIMPLE | employees | NULL | range | idx_name_age_position | idx_name_age_position | 140 | NULL | 1 | 5 | Using index condition | +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+ 1 row in setmysql> EXPLAIN SELECT * FROM employees_2 WHERE name like 'LiLei%' AND age = 22 AND position ='manager'; +----+-------------+-------------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+ | 1 | SIMPLE | employees_2 | NULL | range | idx_name_age_position | idx_name_age_position | 140 | NULL | 1 | 33.33 | Using index condition | +----+-------------+-------------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+ 1 row in set

原因： 索引下推 MySQL -索引下推 Index Condition Pushdown 初探

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特殊例子

一般情況 ，但也不絕對。看下面這個例子

假設你這個表 的name字段 是以Artisan開頭的，從Artisan1 到Artisan100000

再去like的話 ，mysql會基于cost，自主選擇 ，比如如下走了全表掃描。

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搞定MySQL

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