SQL中一些不经意隐式类型转换或者函数使用导致索引失效有关问题
SQL中一些不经意隐式类型转换或者函数使用导致索引失效问题
- SQL code
--A.函数转换 --使用FULL JOIN,以前碰到的环境:有时写视图时将两表合并使用FULL JOIN --示例: DECLARE @A TABLE(NAME VARCHAR(10) NOT NULL PRIMARY KEY,ID INT) INSERT @A SELECT CASE WHEN NUMBER%2=0 THEN 'A'+LTRIM(NUMBER) ELSE 'B'+LTRIM(NUMBER) END , NUMBER FROM MASTER..SPT_VALUES WHERE TYPE='P' AND NUMBER BETWEEN 1 AND 10 DECLARE @B TABLE(NAME VARCHAR(10) NOT NULL PRIMARY KEY,ID INT) INSERT @B SELECT CASE WHEN NUMBER%2=0 THEN 'A'+LTRIM(NUMBER) ELSE 'C'+LTRIM(NUMBER) END, NUMBER FROM MASTER..SPT_VALUES WHERE TYPE='P' AND NUMBER BETWEEN 1 AND 20 --1.先SUM统计后再FULL JOIN 连接(这里没有重复所以没SUM了,测试用) SELECT ISNULL(A.NAME,B.NAME) NAME,A.ID,B.ID FROM @A A FULL JOIN @B B ON A.NAME=B.NAME WHERE (A.NAME='A10' OR B.NAME='A10') SELECT ISNULL(A.NAME,B.NAME) NAME,A.ID,B.ID FROM @A A FULL JOIN @B B ON A.NAME=B.NAME WHERE ISNULL(A.NAME,B.NAME)='A10' --上面两语句逻辑相同,因函数转换导致失效(一般建立好视图后才查询SELECT * FROM V_NAME WHERE NAME=@NAME) --解决办法:使用UNION ALL(上面两个NAME 列类型相同) --2.用UNION ALL后再SUM SELECT NAME,SUM(AID) AID,SUM(BID) FROM ( SELECT NAME,ID AS AID,0 AS BID FROM @A UNION ALL SELECT NAME,0 ,ID FROM @B ) T WHERE NAME='A10' GROUP BY NAME -------------------------------- --B.列类型转换 --1使用UNION ALL,以前遇到的环境如上示例 --示例: SET SHOWPLAN_TEXT ON GO DECLARE @A TABLE(COL VARCHAR(3) NOT NULL PRIMARY KEY) INSERT @A SELECT 'A' UNION ALL SELECT 'B' --列长度 和 @A 不一致 DECLARE @B TABLE(COL VARCHAR(5) NOT NULL PRIMARY KEY) INSERT @B SELECT 'ABCCC' UNION ALL SELECT 'BCFDF' --列长度 和 @A 一致 DECLARE @C TABLE(COL VARCHAR(3) NOT NULL PRIMARY KEY) INSERT @C SELECT 'ADD' UNION ALL SELECT 'BAD' SELECT * FROM ( SELECT COL FROM @A UNION ALL SELECT COL FROM @B )T WHERE COL LIKE 'A%' SELECT * FROM ( SELECT COL FROM @A UNION ALL SELECT COL FROM @C )T WHERE COL LIKE 'A%' GO SET SHOWPLAN_TEXT ON --结果:列类型不一致会导致类型转换,低向高精度转换,最后引起索引失效 GO /* StmtText ---------------------------------------------------------------- SELECT * FROM ( SELECT COL FROM @A UNION ALL SELECT COL FROM @B )T WHERE COL LIKE 'A%' (所影响的行数为 1 行) StmtText -------------------------------------------------------------------- |--Concatenation |--Filter(WHERE:(like([Expr1002], 'A%', NULL))) | |--Compute Scalar(DEFINE:([Expr1002]=Convert(@A.[COL]))) | |--Clustered Index Scan(OBJECT:(@A)) |--Clustered Index Seek(OBJECT:(@B), SEEK:(@B.[COL] >= 'A' AND @B.[COL] < 'B'), WHERE:(like(@B.[COL], 'A%', NULL)) ORDERED FORWARD) (所影响的行数为 5 行) StmtText --------------------------------------------------------------- SELECT * FROM ( SELECT COL FROM @A UNION ALL SELECT COL FROM @C )T WHERE COL LIKE 'A%' (所影响的行数为 1 行) StmtText -------------------------------------------------------------------- |--Concatenation |--Clustered Index Seek(OBJECT:(@A), SEEK:(@A.[COL] >= 'A' AND @A.[COL] < 'B'), WHERE:(like(@A.[COL], 'A%', NULL)) ORDERED FORWARD) |--Clustered Index Seek(OBJECT:(@C), SEEK:(@C.[COL] >= 'A' AND @C.[COL] < 'B'), WHERE:(like(@C.[COL], 'A%', NULL)) ORDERED FORWARD) (所影响的行数为 3 行) StmtText ------------------------ SET SHOWPLAN_TEXT ON */ --2使用SQL语句时数值传参数处理 --示例: SET SHOWPLAN_TEXT ON GO DECLARE @T TABLE(ID DECIMAL(10,2) PRIMARY KEY) INSERT @T SELECT 1.23 UNION ALL SELECT 2.3 DECLARE @PARA DEC(12,2) SET @PARA=2.3 SELECT * FROM @T WHERE ID=@PARA SELECT * FROM @T WHERE ID=CONVERT(DEC(10,2),@PARA) GO SET SHOWPLAN_TEXT OFF GO --结果ID类型向高类型转换,导致索引失效 /* SELECT * FROM @T WHERE ID=@PARA (所影响的行数为 2 行) StmtText ---------------------------- |--Clustered Index Scan(OBJECT:(@T), WHERE:(@T.[ID]=[@PARA])) (所影响的行数为 1 行) StmtText ------------------------------------------------------ SELECT * FROM @T WHERE ID=CONVERT(DEC(10,2),@PARA) (所影响的行数为 1 行) StmtText ---------------------------------------------------- |--Clustered Index Seek(OBJECT:(@T), SEEK:(@T.[ID]=Convert([@PARA])) ORDERED FORWARD) (所影响的行数为 1 行) */ --3使用SQL语句时传字符类型参数处理 --示例: IF OBJECT_ID('T') IS NOT NULL DROP TABLE T CREATE TABLE T(COL CHAR(10) PRIMARY KEY ) GO INSERT T SELECT 'ABCD' GO DECLARE @S VARCHAR(10),@S1 NVARCHAR(10) SELECT @S='ABCD',@S1='ABCD' SELECT * FROM T WHERE COL=@S SELECT * FROM T WHERE COL=@S1 --结果:是类型存储的问题,varchar\char 存储时只占一个字节,而nchar\nvarchar存储时是占两个字节(导致类型转换,索引失效) --(这些从SQL存储引擎可以确定)具体可以参考博客:石头哥的,小麦的,影子老师的 /*StmtText ------------------------------------- DECLARE @S VARCHAR(10),@S1 NVARCHAR(10) SELECT @S='ABCD',@S1='ABCD' SELECT * FROM T WHERE COL=@S (所影响的行数为 2 行) StmtText -------------------------------------------------------------------------- |--Clustered Index Seek(OBJECT:([xzdb].[dbo].[T].[PK__T__1ABFEC2C]), SEEK:([T].[COL]=[@S]) ORDERED FORWARD) (所影响的行数为 1 行) StmtText --------------------------------- SELECT * FROM T WHERE COL=@S1 (所影响的行数为 1 行) StmtText --------------------------------------------------------------------- |--Clustered Index Scan(OBJECT:([xzdb].[dbo].[T].[PK__T__1ABFEC2C]), WHERE:(Convert([T].[COL])=[@S1])) (所影响的行数为 1 行) */ --以上仅学习的一点记录,如有不对之处,希望各位拍砖指导,感谢论坛各位大虾的指点,努力Ing,转载可不注明作者信息