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淺談MySQL中的子查詢優化技巧

編輯：關於MYSQL數據庫

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，你可以點擊這裡，這裡來獲得一些信息，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，然後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：

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；

| 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，將其長時間執行的sql kill掉後，應用恢復正常;
3.分析執行計劃:

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:

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 |
+----+--------------------+------------+-------+-----------------+----------------------+---------+

發現性能還是上不去，關鍵在兩個表掃描的行數並沒有減小（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：

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：

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；