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mysql的子查詢的優(yōu)化一直不是很友好，一直有受業(yè)界批評比較多,也是我在sql優(yōu)化中遇到過最多的問題之一，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(nèi)到外，也就是先完成子查詢的結果，然后在用子查詢來驅(qū)動外查詢的表，完成查詢，但是恰恰相反，子查詢不會先被執(zhí)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：

案例：用戶反饋數(shù)據(jù)庫響應較慢，許多業(yè)務動更新被卡住；登錄到數(shù)據(jù)庫中觀察，發(fā)現(xiàn)長時間執(zhí)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.其他表的更新被阻塞：
update a1 set tradesign='DAB67634-795C-4EAC-B4A0-78F0D531D62F',
markColor=' #CD5555', memotime='2012-09- 22', markPerson='??' where tradeoid in ('gy2012092204495100032') ；
為了盡快恢復應用，將其長時間執(zhí)行的sql kill掉后，應用恢復正常;
3.分析執(zhí)行計劃:
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 |
+----+--------------------+------------+------+---------------+------+---------+------+-------+-----
從執(zhí)行計劃上，我們開始一步一步地進行優(yōu)化：
首先，我們看看執(zhí)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?shù)乃饕?#xff1a;
A.使用覆蓋索引:
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
添加組合索引超過了最大key length限制：
B．查看該表的字段定義：

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．查看表字段的平均長度：

db@3306 ：select max(length(PRONAME)),avg(length(PRONAME)) from a2; +----------------------+----------------------+ | max(length(PRONAME)) | avg(length(PRONAME)) | +----------------------+----------------------+ | 95 | 24.5588 |

D．縮小字段長度

alter table modify column PRONAME varchar(156);

再進行執(zhí)行計劃分析：
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(fā)現(xiàn)性能還是上不去，關鍵在兩個表掃描的行數(shù)并沒有減小（8962*41005），上面添加的索引沒有太大的效果，現(xiàn)在查看t表的執(zhí)行結果：
db@3306 ：select orderdto1_.tradeoid from t orderdto1_ where orderdto1_.proname like '%??%' or orderdto1_.procode like '%??%';
Empty set (0.05 sec)
結果集為空，所以需要將t表的結果集做作為驅(qū)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(lián)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.查看執(zhí)行計劃：
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.執(zhí)行時間：

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)

縮短到了毫秒；

轉(zhuǎn)載于:https://my.oschina.net/sansom/blog/125752

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