淺談MySQL中的子查詢優化技能。本站提示廣大學習愛好者：（淺談MySQL中的子查詢優化技能）文章只能為提供參考，不一定能成為您想要的結果。以下是淺談MySQL中的子查詢優化技能正文
mysql的子查詢的優化一向不是很友愛，一向有受業界批駁比擬多,也是我在sql優化中碰到過最多的成績之一，你可以點擊這裡 ，這裡來取得一些信息，mysql在處置子查詢的時刻，會將子查詢改寫,平日情形下，我們願望由內到外，也就是先完成子查詢的成果，然後在用子查詢來驅動外查詢的表，完成查詢，然則恰好相反，子查詢不會先被履行；明天願望經由過程引見一些現實的案例來加深對mysql子查詢的懂得：
案例：用戶反應數據庫呼應較慢，很多營業動更新被卡住；登錄到數據庫中不雅察，發明長時光履行的sql；
| 10437 | usr0321t9m9 | 10.242.232.50:51201 | oms | Execute | 1179 | Sending
Sql為：
select tradedto0_.* from a1 tradedto0_ where tradedto0_.tradestatus='1' and (tradedto0_.tradeoid in (select orderdto1_.tradeoid from a2 orderdto1_ where orderdto1_.proname like '%??%' or orderdto1_.procode like '%??%')) and tradedto0_.undefine4='1' and tradedto0_.invoicetype='1' and tradedto0_.tradestep='0' and (tradedto0_.orderCompany like '0002%') order by tradedto0_.tradesign ASC, tradedto0_.makertime desc limit 15;
2.其他表的更新被壅塞：
update a1 set tradesign='DAB67634-795C-4EAC-B4A0-78F0D531D62F',
markColor=' #CD5555', memotime='2012-09- 22', markPerson='??' where tradeoid in ('gy2012092204495100032') ；
為了盡快恢復運用，將其長時光履行的sql kill失落後，運用恢復正常;
3.剖析履行籌劃:
db@3306 ：explain select tradedto0_.* from a1 tradedto0_ where tradedto0_.tradestatus='1' and (tradedto0_.tradeoid in (select orderdto1_.tradeoid from a2 orderdto1_ where orderdto1_.proname like '%??%' or orderdto1_.procode like '%??%')) and tradedto0_.undefine4='1' and tradedto0_.invoicetype='1' and tradedto0_.tradestep='0' and (tradedto0_.orderCompany like '0002%') order by tradedto0_.tradesign ASC, tradedto0_.makertime desc limit 15; +----+--------------------+------------+------+---------------+------+---------+------+-------+----- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+--------------------+------------+------+---------------+------+---------+------+-------+----- | 1 | PRIMARY | tradedto0_ | ALL | NULL | NULL | NULL | NULL | 27454 | Using where; Using filesort | | 2 | DEPENDENT SUBQUERY | orderdto1_ | ALL | NULL | NULL | NULL | NULL | 40998 | Using where | +----+--------------------+------------+------+---------------+------+---------+------+-------+-----
從履行籌劃上，我們開端一步一步地停止優化：
起首，我們看看履行籌劃的第二行，也就是子查詢的那部門，orderdto1_停止了全表的掃描，我們看看能不克不及添加恰當的索引：
A.應用籠罩索引:
db@3306：alter table a2 add index ind_a2(proname,procode,tradeoid); ERROR 1071 (42000): Specified key was too long; max key length is 1000 bytes
添加組合索引跨越了最年夜key length限制：
B．檢查該表的字段界說：
db@3306 ：DESC a2 ; +---------------------+---------------+------+-----+---------+-------+ | FIELD | TYPE | NULL | KEY | DEFAULT | Extra | +---------------------+---------------+------+-----+---------+-------+ | OID | VARCHAR(50) | NO | PRI | NULL | | | TRADEOID | VARCHAR(50) | YES | | NULL | | | PROCODE | VARCHAR(50) | YES | | NULL | | | PRONAME | VARCHAR(1000) | YES | | NULL | | | SPCTNCODE | VARCHAR(200) | YES | | NULL | |
C．檢查表字段的均勻長度：
db@3306 ：SELECT MAX(LENGTH(PRONAME)),avg(LENGTH(PRONAME)) FROM a2; +----------------------+----------------------+ | MAX(LENGTH(PRONAME)) | avg(LENGTH(PRONAME)) | +----------------------+----------------------+ | 95 | 24.5588 |
D．減少字段長度
ALTER TABLE MODIFY COLUMN PRONAME VARCHAR(156);
再停止履行籌劃剖析：
db@3306 ：explain select tradedto0_.* from a1 tradedto0_ where tradedto0_.tradestatus='1' and (tradedto0_.tradeoid in (select orderdto1_.tradeoid from a2 orderdto1_ where orderdto1_.proname like '%??%' or orderdto1_.procode like '%??%')) and tradedto0_.undefine4='1' and tradedto0_.invoicetype='1' and tradedto0_.tradestep='0' and (tradedto0_.orderCompany like '0002%') order by tradedto0_.tradesign ASC, tradedto0_.makertime desc limit 15; +----+--------------------+------------+-------+-----------------+----------------------+---------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+--------------------+------------+-------+-----------------+----------------------+---------+ | 1 | PRIMARY | tradedto0_ | ref | ind_tradestatus | ind_tradestatus | 345 | const,const,const,const | 8962 | Using where; Using filesort | | 2 | DEPENDENT SUBQUERY | orderdto1_ | index | NULL | ind_a2 | 777 | NULL | 41005 | Using where; Using index | +----+--------------------+------------+-------+-----------------+----------------------+---------+
發明機能照樣上不去，症結在兩個表掃描的行數並沒有減小（8962*41005），下面添加的索引沒有太年夜的後果，如今檢查t表的履行成果：
db@3306 ：select orderdto1_.tradeoid from t orderdto1_ where orderdto1_.proname like '%??%' or orderdto1_.procode like '%??%'; Empty set (0.05 sec)
成果集為空，所以須要將t表的成果集造作為驅動表；
4．經由過程下面測實驗證，通俗的mysql子查詢寫法機能上是很差的，為mysql的子查詢自然的弱點，須要將sql停止改寫為聯系關系的寫法：
select tradedto0_.* from a1 tradedto0_ ,(select orderdto1_.tradeoid from a2 orderdto1_ where orderdto1_.proname like '%??%' or orderdto1_.procode like '%??%')t2 where tradedto0_.tradestatus='1' and (tradedto0_.tradeoid=t2.tradeoid ) and tradedto0_.undefine4='1' and tradedto0_.invoicetype='1' and tradedto0_.tradestep='0' and (tradedto0_.orderCompany like '0002%') order by tradedto0_.tradesign ASC, tradedto0_.makertime desc limit 15;
5.檢查履行籌劃：
db@3306 ：explain select tradedto0_.* from a1 tradedto0_ ,(select orderdto1_.tradeoid from a2 orderdto1_ where orderdto1_.proname like '%??%' or orderdto1_.procode like '%??%')t2 where tradedto0_.tradestatus='1' and (tradedto0_.tradeoid=t2.tradeoid ) and tradedto0_.undefine4='1' and tradedto0_.invoicetype='1' and tradedto0_.tradestep='0' and (tradedto0_.orderCompany like '0002%') order by tradedto0_.tradesign ASC, tradedto0_.makertime desc limit 15; +----+-------------+------------+-------+---------------+----------------------+---------+------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+-------+---------------+----------------------+---------+------+ | 1 | PRIMARY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE noticed after reading const tables | | 2 | DERIVED | orderdto1_ | index | NULL | ind_a2 | 777 | NULL | 41005 | Using where; Using index | +----+-------------+------------+-------+---------------+----------------------+---------+------+
6.履行時光：
db@3306 ：select tradedto0_.* from a1 tradedto0_ ,(select orderdto1_.tradeoid from a2 orderdto1_ where orderdto1_.proname like '%??%' or orderdto1_.procode like '%??%')t2 where tradedto0_.tradestatus='1' and (tradedto0_.tradeoid=t2.tradeoid ) and tradedto0_.undefine4='1' and tradedto0_.invoicetype='1' and tradedto0_.tradestep='0' and (tradedto0_.orderCompany like '0002%') order by tradedto0_.tradesign ASC, tradedto0_.makertime desc limit 15; Empty set (0.03 sec)
延長到了毫秒；
