讓天下沒有難用的資料庫 » 淺談mysql的子查詢
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，你可以點選這裡 ，這裡來獲得一些資訊，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：使用者反饋資料庫響應較慢，許多業務動更新被卡住；登入到資料庫中觀察，發現長時間執行的sql；
| 10437 | usr0321t9m9 | 10.242.232.50:51201 | oms | Execute | 1179
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') ；為了儘快恢復應用，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:
[email protected] ：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，我們看看能不能新增適當的索引：
A.使用覆蓋索引:
[email protected]：alter table a2 add index ind_a2(proname,procode,tradeoid);
ERROR 1071 (42000): Specified key was too long; max key length is 1000 bytes新增組合索引超過了最大key length限制：
B．檢視該表的欄位定義：
[email protected] ：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．查看錶欄位的平均長度：
[email protected] ：SELECT MAX(LENGTH(PRONAME)),avg(LENGTH(PRONAME)) FROM a2; +----------------------+----------------------+ | MAX(LENGTH(PRONAME)) | avg(LENGTH(PRONAME)) | +----------------------+----------------------+ | 95 | 24.5588 |
D．縮小欄位長度
ALTER TABLE MODIFY COLUMN PRONAME VARCHAR(156);
再進行執行計劃分析：
[email protected] ：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 |
+----+--------------------+------------+-------+-----------------+----------------------+---------+
發現效能還是上不去，關鍵在兩個表掃描的行數並沒有減小（8962*41005），上面新增的索引沒有太大的效果，現在檢視t表的執行結果：
[email protected] ：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子查詢寫法效能上是很差的，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.檢視執行計劃：
[email protected] ：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：
[email protected] ：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；
總結：
1. mysql子查詢在執行計劃上有著明顯的弱點，需要將子查詢進行改寫
可以參考：
a. 生產庫中遇到mysql的子查詢：http://hidba.org/?p=412
b. 內建的builtin InnoDB,子查詢阻塞更新：http://hidba.org/?p=456
2. 在表結構設計上，不要隨便使用varchar(N)的大欄位，導致無法使用索引
可以參考：
a. JDBC記憶體管理—varchar2(4000)的影響：http://hidba.org/?p=31
b. innodb中大欄位的限制：http://hidba.org/?p=144
c. innodb使用大欄位text，blob的一些優化建議： http://hidba.org/?p=551