[TOC] #### 1. 慢查詢介紹 ---- 常見面試題:如何從一個大項(xiàng)目中,迅速定位執(zhí)行速度慢的 SQL 語句? 此時可以使用慢查詢定位執(zhí)行較慢 SQL 語句 慢查詢就是在日志中記錄運(yùn)行比較慢的 SQL 語句,是指所有執(zhí)行超過 long_query_time 參數(shù)設(shè)定的時間閾值的 SQL 語句查詢 #### 2. 慢查詢狀態(tài) --- ``` # 查看是否開啟慢查詢?nèi)罩?ON 1 開啟 OFF 0 關(guān)閉(默認(rèn)是關(guān)閉的) show variables like 'slow_query_log'; # 開啟慢查詢?nèi)罩?set global slow_query_log = 1; # 關(guān)閉慢查詢?nèi)罩?set global slow_query_log = 0; ``` ![](https://img.itqaq.com/art/content/79a83d51631c6d720a3f5945d6aaf735.png) 使用下面命令開啟慢查詢時,當(dāng)重啟 mysql 服務(wù)后,慢查詢會恢復(fù)到默認(rèn)的關(guān)閉狀態(tài) ``` set global slow_query_log = 1; ``` 慢查詢狀態(tài)想要永久生效,就必須修改配置文件 my.conf,將下面配置項(xiàng)放入配置文件 my.conf 中,重啟 mysql 服務(wù)即可生效 ``` slow_query_log = 1 ``` ![](https://img.itqaq.com/art/content/e4d1c83f0f580f3d5f99dec9c1e862ad.png) #### 3. 慢查詢時間閾值 --- 上面在介紹慢查詢時,已經(jīng)說明慢查詢?nèi)罩局杏涗浀氖浅鰰r間閾值的 SQL 語句 這個時間閾值可以通過下面這個命令查看,默認(rèn)是 10 秒 ``` # 查看當(dāng)前慢查詢時間閾值 show variables like 'long_query_time'; # 臨時修改慢查詢時間閾值 (1 秒) set long_query_time = 1; ``` ![](https://img.itqaq.com/art/content/9d422dfc4a882ef51d68d8b61dcd1a14.png) 在配置文件 my.conf 中添加配置項(xiàng),使設(shè)置的時間閾值永久生效 ``` long_query_time = 2 ``` ![](https://img.itqaq.com/art/content/888897b0fea8ef5c01e891a74aed0710.png) #### 4. 構(gòu)建大表,測試慢查詢 --- 創(chuàng)建一個數(shù)據(jù)庫,執(zhí)行下面 SQL ``` CREATE TABLE dept( /*部門表*/ deptno MEDIUMINT UNSIGNED NOT NULL DEFAULT 0, /*編號*/ dname VARCHAR(20) NOT NULL DEFAULT "", /*名稱*/ loc VARCHAR(13) NOT NULL DEFAULT "" /*地點(diǎn)*/ ) ENGINE=MyISAM DEFAULT CHARSET=utf8 ; CREATE TABLE emp( empno MEDIUMINT UNSIGNED NOT NULL DEFAULT 0, /*編號*/ ename VARCHAR(20) NOT NULL DEFAULT "", /*名字*/ job VARCHAR(9) NOT NULL DEFAULT "",/*工作*/ mgr MEDIUMINT UNSIGNED NOT NULL DEFAULT 0,/*上級編號*/ hiredate DATE NOT NULL,/*入職時間*/ sal DECIMAL(7,2) NOT NULL,/*薪水*/ comm DECIMAL(7,2) NOT NULL,/*紅利*/ deptno MEDIUMINT UNSIGNED NOT NULL DEFAULT 0 /*部門編號*/ )ENGINE=MyISAM DEFAULT CHARSET=utf8 ; CREATE TABLE salgrade ( grade MEDIUMINT UNSIGNED NOT NULL DEFAULT 0, losal DECIMAL(17,2) NOT NULL, hisal DECIMAL(17,2) NOT NULL )ENGINE=MyISAM DEFAULT CHARSET=utf8; -- 測試數(shù)據(jù) INSERT INTO salgrade VALUES (1,700,1200); INSERT INTO salgrade VALUES (2,1201,1400); INSERT INTO salgrade VALUES (3,1401,2000); INSERT INTO salgrade VALUES (4,2001,3000); INSERT INTO salgrade VALUES (5,3001,9999); # 創(chuàng)建存儲過程 delimiter $$ create function rand_string(n INT) returns varchar(255) #該函數(shù)會返回一個字符串 begin #chars_str定義一個變量 chars_str,類型是 varchar(100),默認(rèn)值’abcdefghijklmnopqrstuvwxyzABCDEFJHIJKLMNOPQRSTUVWXYZ’; declare chars_str varchar(100) default 'abcdefghijklmnopqrstuvwxyzABCDEFJHIJKLMNOPQRSTUVWXYZ'; declare return_str varchar(255) default ''; declare i int default 0; while i < n do set return_str =concat(return_str,substring(chars_str,floor(1+rand()*52),1)); set i = i + 1; end while; return return_str; end $$ create procedure insert_emp2(in start int(10),in max_num int(10)) begin declare i int default 0; #set autocommit =0 把a(bǔ)utocommit設(shè)置成0 set autocommit = 0; repeat set i = i + 1; insert into emp values ((start+i) ,rand_string(6),'SALESMAN',0001,curdate(),2000,400,floor(rand()*5+5)); until i = max_num end repeat; commit; end $$ delimiter ; # 調(diào)用剛剛寫好的函數(shù), 1800000條記錄,從100001號開始 call insert_emp2(100001,4000000); ``` 查詢 emp 表數(shù)據(jù),執(zhí)行 1.55 秒 ``` mysql> select * from emp; +--------+--------+----------+-----+------------+---------+--------+--------+ | empno | ename | job | mgr | hiredate | sal | comm | deptno | +--------+--------+----------+-----+------------+---------+--------+--------+ | 100002 | BzVfMu | SALESMAN | 1 | 2022-05-15 | 2000.00 | 400.00 | 8 | | 100003 | wcUtjQ | SALESMAN | 1 | 2022-05-15 | 2000.00 | 400.00 | 7 | | ... | ... | ... | ... | ... | ... | ... | ... | +--------+--------+----------+-----+------------+---------+--------+--------+ 4000000 rows in set (1.55 sec) ``` ``` # 查看慢查詢?nèi)罩疚募娣盼恢?show variables like 'slow_query_log_file'; ``` 設(shè)置 long_query_time = 1,此時會生成慢查詢?nèi)罩?,文件?nèi)容如下所示 ``` # Time: 2022-05-15T02:31:37.897427Z # User@Host: root[root] @ localhost [] Id: 2 # Query_time: 1.548641 Lock_time: 0.000156 Rows_sent: 4000000 Rows_examined: 4000000 SET timestamp=1652581897; select * from emp; ```