阿里規(guī)定超過(guò)三張表禁止join，為啥？

作者：e71hao

blog.itpub.net/30393770/viewspace-2650450/

一、 問(wèn)題提出

《阿里巴巴JAVA開(kāi)發(fā)手冊(cè)》里面寫(xiě)超過(guò)三張表禁止join，這是為什么？

二、問(wèn)題分析

對(duì)這個(gè)結(jié)論，你是否有懷疑呢？也不知道是哪位先哲說(shuō)的不要人云亦云，今天我設(shè)計(jì)sql，來(lái)驗(yàn)證這個(gè)結(jié)論。（實(shí)驗(yàn)沒(méi)有從代碼角度分析，目前達(dá)不到。可以把mysql當(dāng)一個(gè)黑盒，使用角度來(lái)驗(yàn)證這個(gè)結(jié)論） 驗(yàn)證結(jié)論的時(shí)候，會(huì)有很多發(fā)現(xiàn)，各位往后看。

三、 實(shí)驗(yàn)環(huán)境

vmware10+centos7.4+mysql5.7.22

• centos7內(nèi)存4.5G，4核，50G硬盤(pán)。

• mysql配置為2G，特別說(shuō)明硬盤(pán)是SSD。

四、我概述下我的實(shí)驗(yàn)

有4張表，student學(xué)生表，teacher老師表，course課程表，sc中間關(guān)系表，記錄了學(xué)生選修課程以及分?jǐn)?shù)。具體sql腳本，看文章結(jié)尾，我附上。中間我自己寫(xiě)了造數(shù)據(jù)的腳本，也在結(jié)尾。

實(shí)驗(yàn)是為解決一個(gè)問(wèn)題的：查詢(xún)選修“tname553”老師所授課程的學(xué)生中，成績(jī)最高的學(xué)生姓名及其成績(jī)。

查詢(xún)sql是：

select?Student.Sname,course.cname,score?
????from?Student,SC,Course?,Teacher?
????where?Student.s_id=SC.s_id?and?SC.c_id=Course.c_id??and?sc.t_id=teacher.t_id?
????and?Teacher.Tname='tname553'?
????and?SC.score=(select?max(score)from?SC?where?sc.t_id=teacher.t_Id);

我來(lái)分析一下這個(gè)語(yǔ)句：4張表等值join，還有一個(gè)子查詢(xún)。算是比較簡(jiǎn)單的sql語(yǔ)句了（相比ERP動(dòng)就10張表的哦，已經(jīng)很簡(jiǎn)單了）。我 還會(huì)分解這個(gè)語(yǔ)句成3個(gè)簡(jiǎn)單的sql：

?select?max(score)??from?SC?,Teacher?where?sc.t_id=teacher.t_Id?and?Teacher.Tname='tname553';
???select?sc.t_id,sc.s_id,score???from?SC?,Teacher?
???where?sc.t_id=teacher.t_Id?
???and?score=590??
???and?Teacher.Tname='tname553';
????select?Student.Sname,course.cname,score?
????from?Student,SC?,course
????where?Student.s_id=SC.s_id?and??sc.s_id?in?(20769800,48525000,26280200)?and?course.c_id?=?sc.c_id;

我來(lái)分析下：第一句，就是查詢(xún)最高分，得到最高分590分。第二句就是查詢(xún)出最高分的學(xué)生id，得到

20769800,48525000,26280200

第三句就是查詢(xún)出學(xué)生名字和分?jǐn)?shù)。這樣這3個(gè)語(yǔ)句的就可以查詢(xún)出來(lái) 成績(jī)最高的學(xué)生姓名及其成績(jī) 。

接下來(lái)我會(huì)分別造數(shù)據(jù)：1千萬(wàn)選課記錄(一個(gè)學(xué)生選修2門(mén)課),造500萬(wàn)學(xué)生，100萬(wàn)老師(一個(gè)老師帶5個(gè)學(xué)生，挺高端的吧)，1000門(mén)課，。用上面查詢(xún)語(yǔ)句查詢(xún)。其中sc表我測(cè)試了下有索引和沒(méi)有索引情況，具體見(jiàn)下表。

再接下來(lái)，我會(huì)造1億選課記錄(一個(gè)學(xué)生選修2門(mén)課),5000萬(wàn)學(xué)生，1000萬(wàn)老師，1000門(mén)課。然后分別執(zhí)行上述語(yǔ)句。最后我會(huì)在oracle數(shù)據(jù)庫(kù)上執(zhí)行上述語(yǔ)句。

五、下面兩張表是測(cè)試結(jié)果

六、仔細(xì)看上表，可以發(fā)現(xiàn)：

1.步驟3.1沒(méi)有在連接鍵上加索引，查詢(xún)很慢，說(shuō)明：“多表關(guān)聯(lián)查詢(xún)時(shí)，保證被關(guān)聯(lián)的字段需要有索引”；

2.步驟6.1,6.2,6.3，換成簡(jiǎn)單sql，在數(shù)據(jù)量1億以上， 查詢(xún)時(shí)間還能勉強(qiáng)接受。此時(shí)說(shuō)明mysql查詢(xún)有些吃力了，但是仍然嫩查詢(xún)出來(lái)。

3.步驟5.1，mysql查詢(xún)不出來(lái)，4表連接，對(duì)我本機(jī)mysql來(lái)說(shuō)，1.5億數(shù)據(jù)超過(guò)極限了（我調(diào)優(yōu)過(guò)這個(gè)SQL，執(zhí)行計(jì)劃和索引都走了，沒(méi)有問(wèn)題，show profile顯示在sending data.這個(gè)問(wèn)題另外文章詳談。）

4.對(duì)比1.1 和5.1 步驟sql查詢(xún)，4表連接，對(duì)我本機(jī)mysql來(lái)說(shuō) ，1.5千萬(wàn)數(shù)據(jù)查詢(xún)很流利，是一個(gè)mysql數(shù)據(jù)量流利分水嶺。(這個(gè)只是現(xiàn)象，不太準(zhǔn)確，需要同時(shí)計(jì)算表的容量)。

5.步驟5.1對(duì)比6.1,6.2,6.3，多表join對(duì)mysql來(lái)說(shuō)，處理有些吃力。

6.超過(guò)三張表禁止join,這個(gè)規(guī)則是針對(duì)mysql來(lái)說(shuō)的。后續(xù)會(huì)看到我用同樣機(jī)器，同樣數(shù)據(jù)量，同樣內(nèi)存，可以完美計(jì)算 1.5億數(shù)據(jù)量join。針對(duì)這樣一個(gè)規(guī)則，對(duì)開(kāi)發(fā)來(lái)說(shuō) ，需要把一些邏輯放到應(yīng)用層去查詢(xún)。

總結(jié)：?這個(gè)規(guī)則 超過(guò)三張表禁止join ,由于數(shù)據(jù)量太大的時(shí)候，mysql根本查詢(xún)不出來(lái)，導(dǎo)致阿里出了這樣一個(gè)規(guī)定。(其實(shí)如果表數(shù)據(jù)量少，10張表也不成問(wèn)題,你自己可以試試)而我們公司支付系統(tǒng)朝著大規(guī)模高并發(fā)目標(biāo)設(shè)計(jì)的，所以，遵循這個(gè)規(guī)定。

在業(yè)務(wù)層面來(lái)講，寫(xiě)簡(jiǎn)單sql，把更多邏輯放到應(yīng)用層，我的需求我會(huì)更了解，在應(yīng)用層實(shí)現(xiàn)特定的join也容易得多。

七、讓我們來(lái)看看oracle數(shù)據(jù)庫(kù)的優(yōu)秀表現(xiàn)：

看步驟7.1，就是沒(méi)有索引，join表很多的情況下，oracle仍然26秒查詢(xún)出結(jié)果來(lái)。所以我會(huì)說(shuō)mysql的join很弱。那么問(wèn)題來(lái)了，為什么現(xiàn)在使用很多人使用mysql呢？這是另外一個(gè)問(wèn)題，我會(huì)另外說(shuō)下我的思考。

看完本篇文章，另外我還附加贈(zèng)送，所謂摟草打兔子。就是快速造數(shù)據(jù)。你可以自己先寫(xiě)腳本造數(shù)據(jù)，看看我是怎么造數(shù)據(jù)的，就知道我的技巧了。

八、附上部分截圖

九、附上sql語(yǔ)句和造數(shù)據(jù)腳本

use?stu;
drop?table?if?exists?student;
create?table?student?
??(??s_id?int(11)?not?null?auto_increment?,
?????sno????int(11),?
?????sname?varchar(50),?
?????sage??int(11),?
?????ssex??varchar(8)?,
?????father_id?int(11),
??????mather_id?int(11),
??????note?varchar(500),
?????primary?key?(s_id),
???unique?key?uk_sno?(sno)
??)?engine=innodb?default?charset=utf8mb4;
truncate?table?student;
??delimiter?$$
drop?function?if?exists???insert_student_data?$$
create?function?insert_student_data()
?returns??int?deterministic
????begin
????declare??i?int;
??????set?i=1;
??????while??i<50000000?do?
??????insert?into?student??values(i?,i,?concat('name',i),i,case?when?floor(rand()*10)%2=0?then?'f'?else?'m'?end,floor(rand()*100000),floor(rand()*1000000),concat('note',i)?);
??????set?i=i+1;
??????end?while;
??????return?1;
????end$$
delimiter?;????
select??insert_student_data();
select?count(*)?from?student;
use?stu;
create?table?course?
??(?
?????c_id?int(11)?not?null?auto_increment?,
?????cname?varchar(50)
?????note?varchar(500),?primary?key?(c_id)
??)??engine=innodb?default?charset=utf8mb4;
truncate?table?course;
??delimiter?$$
drop?function?if?exists???insert_course_data?$$
create?function?insert_course_data()
?returns??int?deterministic
????begin
????declare??i?int;
??????set?i=1;
??????while??i<=1000?do?
??????insert?into?course??values(i?,?concat('course',i),floor(rand()*1000),concat('note',i)?);
??????set?i=i+1;
??????end?while;
??????return?1;
????end$$
delimiter?;????
select??insert_course_data();
select?count(*)?from?course;
use?stu;
drop?table?if?exists?sc;
create?table?sc?
??(?
?????s_id????int(11),?
?????c_id????int(11),?
?????t_id????int(11),
?????score?int(11)?
??)??engine=innodb?default?charset=utf8mb4;
truncate?table?sc;
??delimiter?$$
drop?function?if?exists???insert_sc_data?$$
create?function?insert_sc_data()
?returns??int?deterministic
????begin
????declare??i?int;
??????set?i=1;
??????while??i<=50000000?do?
??????insert?into?sc??values(?i,floor(rand()*1000),floor(rand()*10000000),floor(rand()*750))?;
??????set?i=i+1;
??????end?while;
??????return?1;
????end$$
delimiter?;????
select??insert_sc_data();
commit;
select??insert_sc_data();
commit;
create?index?idx_s_id??on?sc(s_id)???;?
create?index?idx_t_id??on?sc(t_id)???;?
create?index?idx_c_id??on?sc(c_id)???;?
select?count(*)?from?sc;
use?stu;
drop?table?if?exists?teacher;
create?table?teacher?
??(?
????t_id??int(11)?not?null?auto_increment?,
?????tname?varchar(50)?,
?????note?varchar(500),primary?key?(t_id)
??)??engine=innodb?default?charset=utf8mb4;

??truncate?table?teacher;
??delimiter?$$
drop?function?if?exists???insert_teacher_data?$$
create?function?insert_teacher_data()
?returns??int?deterministic
????begin
????declare??i?int;
??????set?i=1;
??????while??i<=10000000?do?
??????insert?into?teacher??values(i?,?concat('tname',i),concat('note',i)?);
??????set?i=i+1;
??????end?while;
??????return?1;
????end$$
delimiter?;????
select??insert_teacher_data();
commit;
select?count(*)?from?teacher;

這個(gè)是oracle的測(cè)試和造數(shù)據(jù)腳本

create?tablespace?scott_data??datafile??'/home/oracle/oracle_space/sitpay1/scott_data.dbf'??size?1024m?autoextend?on;?
create?tablespace?scott_index???datafile??'/home/oracle/oracle_space/sitpay1/scott_index.dbf'??size?64m??autoextend?on;?
create?temporary?tablespace?scott_temp??tempfile??'/home/oracle/oracle_space/sitpay1/scott_temp.dbf'??size?64m?autoextend?on;?
drop?user??scott?cascade;
create?user??scott??identified?by??tiger??default?tablespace?scott_data??temporary?tablespace?scott_temp??;
grant?resource,connect,dba?to??scott;
drop?table?student;
create?table?student??
??(??s_id?number(11)?,
?????sno????number(11)?,?
?????sname?varchar2(50),?
?????sage??number(11),?
?????ssex??varchar2(8)?,
?????father_id?number(11),
??????mather_id?number(11),
??????note?varchar2(500)
??)?nologging;
truncate?table?student;
create?or?replace?procedure?insert_student_data
?is?
???q?number(11);
????begin?
?????q:=0;
??????for?i?in??1..50?loop?
??????insert?/*+append*/?into?student???select?rownum+q?as?s_id,rownum+q??as?sno,?concat('sutdent',rownum+q?)?as?sname,floor(dbms_random.value(1,100))?as?sage,'f'?as?ssex,rownum+q??as?father_id,rownum+q??as?mather_id,concat('note',rownum+q?)?as?note?from?dual?connect?by?level<=1000000;
??????q:=q+1000000;
??????commit;
??????end?loop;?
end?insert_student_data;
/
call?insert_student_data();
alter?table?student??add?constraint??pk_student?primary?key?(s_id);
commit;????
select?count(*)?from?student;
create?table?course?
??(?
?????c_id?number(11)?primary?key,
?????cname?varchar2(50),
?????note?varchar2(500)?
??)??;
truncate?table?course;
?create?or?replace?procedure?insert_course_data
?is?
???q?number(11);
????begin?

??????for?i?in??1..1000?loop?
??????insert?/*+append*/?into?course??values(i?,?concat('name',i),concat('note',i)?);??????
??????end?loop;?
end?insert_course_data;
/
call?insert_course_data();
commit;????
select?count(*)?from?course;
create?table?sc?
??(?
?????s_id????number(11),?
?????c_id????number(11),?
?????t_id????number(11),
?????score?number(11)?
??)?nologging;
truncate?table?sc;
?create?or?replace?procedure?insert_sc_data
?is?
???q?number(11);
????begin?
?????q:=0;
??????for?i?in??1..50?loop?
??????insert?/*+append*/?into?sc???select?rownum+q?as?s_id,?floor(dbms_random.value(0,1000))??as?c_id,floor(dbms_random.value(0,10000000))?t_id,floor(dbms_random.value(0,750))?as?score?from?dual?connect?by?level<=1000000;
??????q:=q+1000000;
??????commit;
??????end?loop;?
end?insert_sc_data;
/
call?insert_sc_data();
create?index?idx_s_id??on?sc(s_id)???;?
create?index?idx_t_id??on?sc(t_id)???;?
create?index?idx_c_id??on?sc(c_id)???;?
select?count(*)?from?sc;
create?table?teacher?
??(?
????t_id??number(11)?,
?????tname?varchar2(50)?,
?????note?varchar2(500)
??)nologging?;
????truncate?table?teacher;
create?or?replace?procedure?insert_teacher_data
?is?
???q?number(11);
????begin?
?????q:=0;
??????for?i?in??1..10?loop?
??????insert?/*+append*/?into?teacher???select?rownum+q?as?t_id,?concat('teacher',rownum+q?)?as?tname,concat('note',rownum+q?)?as?note?from?dual?connect?by?level<=1000000;
??????q:=q+1000000;
??????commit;
??????end?loop;?
end?insert_teacher_data;
/
call?insert_teacher_data();
alter?table?teacher??add?constraint??pk_teacher?primary?key?(t_id);
select?count(*)?from?teacher;