《MYSQL教程淺談MySQL中的子查詢優化技巧》要點：
本文介紹了MYSQL教程淺談MySQL中的子查詢優化技巧，希望對您有用。如果有疑問，可以聯系我們。
MYSQL應用mysql的子查詢的優化一直不是很友好,一直有受業界批評比較多,也是我在sql優化中遇到過最多的問題之一,你可以點擊這里 ,這里來獲得一些信息,mysql在處理子查詢的時候,會將子查詢改寫,通常情況下,我們希望由內到外,也就是先完成子查詢的結果,然后在用子查詢來驅動外查詢的表,完成查詢,但是恰恰相反,子查詢不會先被執行；今天希望通過介紹一些實際的案例來加深對mysql子查詢的理解：
MYSQL應用案例：用戶反饋數據庫響應較慢,許多業務動更新被卡住；登錄到數據庫中觀察,發現長時間執行的sql；
MYSQL應用
| 10437 | usr0321t9m9 | 10.242.232.50:51201 | oms | Execute | 1179 | Sending
MYSQL應用Sql為：
MYSQL應用
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;
MYSQL應用2.其他表的更新被阻塞：
MYSQL應用
update a1 set tradesign='DAB67634-795C-4EAC-B4A0-78F0D531D62F',
markColor=' #CD5555', memotime='2012-09- 22', markPerson='??' where tradeoid in ('gy2012092204495100032') ；
MYSQL應用為了盡快恢復應用,將其長時間執行的sql kill掉后,應用恢復正常;
3.分析執行計劃:
MYSQL應用
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 |
+----+--------------------+------------+------+---------------+------+---------+------+-------+-----
MYSQL應用從執行計劃上,我們開始一步一步地進行優化：
首先,我們看看執行計劃的第二行,也就是子查詢的那部分,orderdto1_進行了全表的掃描,我們看看能不能添加適當的索引：
A.使用覆蓋索引:
MYSQL應用
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
MYSQL應用添加組合索引超過了最大key length限制：
B．查看該表的字段定義：
MYSQL應用
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 | |
MYSQL應用C．查看表字段的平均長度：
MYSQL應用
db@3306 ：SELECT MAX(LENGTH(PRONAME)),avg(LENGTH(PRONAME)) FROM a2;
+----------------------+----------------------+
| MAX(LENGTH(PRONAME)) | avg(LENGTH(PRONAME)) |
+----------------------+----------------------+
| 95 | 24.5588 |
MYSQL應用D．縮小字段長度
MYSQL應用
ALTER TABLE MODIFY COLUMN PRONAME VARCHAR(156);
MYSQL應用再進行執行計劃分析：
MYSQL應用
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 |
+----+--------------------+------------+-------+-----------------+----------------------+---------+
MYSQL應用發現性能還是上不去,關鍵在兩個表掃描的行數并沒有減小(8962*41005),上面添加的索引沒有太大的效果,現在查看t表的執行結果：
MYSQL應用
db@3306 ：select orderdto1_.tradeoid from t orderdto1_ where orderdto1_.proname like '%??%' or orderdto1_.procode like '%??%';
Empty set (0.05 sec)
MYSQL應用結果集為空,所以需要將t表的結果集做作為驅動表；
4．通過上面測試驗證,普通的mysql子查詢寫法性能上是很差的,為mysql的子查詢天然的弱點,需要將sql進行改寫為關聯的寫法：
MYSQL應用
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;
MYSQL應用5.查看執行計劃：
MYSQL應用
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 |
+----+-------------+------------+-------+---------------+----------------------+---------+------+
MYSQL應用6.執行時間：
MYSQL應用
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)
MYSQL應用縮短到了毫秒；
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