个人学习笔记13之--游标薄见

浅谈 游标.
这个东西很难说,存在必有其存在的道理.不能一棍子说它怎么不好,效率多差,它还是有适合用的地方,不然微软也不会把它做出来让大家用.这
本身就说明他是个不可舍去的东西..根据书上,我说说游标的使用

1.游标为什么这么受'弃用'



a.游标和关系模式的主要前提是冲突的。关系模式是一个集合的逻辑,它注重的你在执行代码后获取什么’。
但是游标是一种过程的逻辑,它注重的是'如何处理数据'.

b.这个游标逐行操作产生的行操作开销是比较巨大的，一般他的速度也比集合思想处理的慢.(但是有些情况下的数据分布也可能决定游标要快)

c.这个游标要强制我们的优化器执行固定的计划.不是像集合的解决方法那样选一个比较优秀的执行计划.


举个例子说明游标的开销: 我们就做扫描表的操作.实验成 先把SSMS的执行后放弃结果选项后再执行代码。
这样可以保证屏蔽生成输出浪费的时间,保证我们的2个操作时间差就差在性能上,体现游标的开销

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---1.先插入1000000条数据进入T1表,(这里的NUMS表是一个辅助表,我前面的学习笔记写过很多方法创建这个临时表)
SET NOCOUNT ON;
USE tempdb;
GO
IF OBJECT_ID('dbo.T1') IS NOT NULL
DROP TABLE dbo.T1;
GO
SELECT n AS keycol, CAST('a' AS CHAR(200)) AS filler
INTO dbo.T1
FROM dbo.Nums;
---2.在字段上建立索引
CREATE UNIQUE CLUSTERED INDEX idx_keycol ON dbo.T1(keycol)
---3.清空缓存
DBCC DROPCLEANBUFFERS;
GO
---4.执行这条记录2次，一定要选中放弃执行结果选项(工具|选项|查询结果|SQL SERVER|以网格或者文本显示结果)里有这个选项
--第一次运行把数据加载至内存,书上叫冷缓存,第二次叫热缓存
SELECT keycol, filler FROM dbo.T1;
---1.先插入1000000条数据进入T1表,(这里的NUMS表是一个辅助表,我前面的学习笔记写过很多方法创建这个临时表)
SET NOCOUNT ON;
USE tempdb;
GO
IF OBJECT_ID('dbo.T1') IS NOT NULL
DROP TABLE dbo.T1;
GO
SELECT n AS keycol, CAST('a' AS CHAR(200)) AS filler
INTO dbo.T1
FROM dbo.Nums;
---2.在字段上建立索引
CREATE UNIQUE CLUSTERED INDEX idx_keycol ON dbo.T1(keycol)
---3.清空缓存
DBCC DROPCLEANBUFFERS;
GO
---4.执行这条记录2次，一定要选中放弃执行结果选项(工具|选项|查询结果|SQL SERVER|以网格或者文本显示结果)里有这个选项
--第一次运行把数据加载至内存,书上叫冷缓存,第二次叫热缓存
SELECT keycol, filler FROM dbo.T1;


----我这机器第一次是7秒,第二次是1秒

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---5.再次清空缓存
DBCC DROPCLEANBUFFERS;
GO
---6.运行如下游标2次
DECLARE @keycol AS INT, @filler AS CHAR(200);
DECLARE C CURSOR FAST_FORWARD FOR SELECT keycol, filler FROM dbo.T1;
OPEN C
FETCH NEXT FROM C INTO @keycol, @filler;
WHILE @@fetch_status = 0
BEGIN
FETCH NEXT FROM C INTO @keycol, @filler;
END
CLOSE C;
DEALLOCATE C;
GO
---5.再次清空缓存
DBCC DROPCLEANBUFFERS;
GO
---6.运行如下游标2次
DECLARE @keycol AS INT, @filler AS CHAR(200);
DECLARE C CURSOR FAST_FORWARD FOR SELECT keycol, filler FROM dbo.T1;
OPEN C
FETCH NEXT FROM C INTO @keycol, @filler;
WHILE @@fetch_status = 0
BEGIN
FETCH NEXT FROM C INTO @keycol, @filler;
END
CLOSE C;
DEALLOCATE C;
GO





----我的机器上一次22秒,一次17秒.



------我们分析结果 只考虑热缓存,第二次没写入内存 这样就不涉及物理的I/O操作,你自己也看到了 我这赤裸裸的17倍速度.(每个人机器上不一样)
---但是可以肯定 集合的比游标要快很多倍.

PS；这里还有个三不像的解法.看起来像游标,其实基于集合.（一定要选中放弃执行结果选项）
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DECLARE @keycol AS INT, @filler AS CHAR(200);

SELECT @keycol = keycol, @filler = filler
FROM (SELECT TOP (1) keycol, filler
FROM dbo.T1
ORDER BY keycol) AS D;

WHILE @@rowcount = 1
BEGIN

SELECT @keycol = keycol, @filler = filler
FROM (SELECT TOP (1) keycol, filler
FROM dbo.T1
WHERE keycol > @keycol
ORDER BY keycol) AS D;
END
GO
DECLARE @keycol AS INT, @filler AS CHAR(200);

SELECT @keycol = keycol, @filler = filler
FROM (SELECT TOP (1) keycol, filler
FROM dbo.T1
ORDER BY keycol) AS D;

WHILE @@rowcount = 1
BEGIN

SELECT @keycol = keycol, @filler = filler
FROM (SELECT TOP (1) keycol, filler
FROM dbo.T1
WHERE keycol > @keycol
ORDER BY keycol) AS D;
END
GO

---这个方法在我机器上第一次运行了20秒,第二次是14秒(- - || 我很郁闷 ,书上说要比游标还慢好几倍..可是测试来测试就是差不多嘛)

d.我们这个游标的代码还不是很长,这还看不出游标的代码还需要维护的成本,但是往往游标非常长的,那是需要成本的.


2.游标也有受'亲赖'的时候
当问题本身就是基于顺序的时候,游标是很有可能比基于集合的代码更加快(但不是一定快)

a.自定义的一些聚合



--首先插入数据
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USE tempdb;
GO
IF OBJECT_ID('dbo.Groups') IS NOT NULL
DROP TABLE dbo.Groups;
GO

CREATE TABLE dbo.Groups
(
groupid VARCHAR(10) NOT NULL,
memberid INT NOT NULL,
string VARCHAR(10) NOT NULL,
val INT NOT NULL,
PRIMARY KEY (groupid, memberid)
);

INSERT INTO dbo.Groups(groupid, memberid, string, val)
VALUES('a', 3, 'stra1', 6);
INSERT INTO dbo.Groups(groupid, memberid, string, val)
VALUES('a', 9, 'stra2', 7);
INSERT INTO dbo.Groups(groupid, memberid, string, val)
VALUES('b', 2, 'strb1', 3);
INSERT INTO dbo.Groups(groupid, memberid, string, val)
VALUES('b', 4, 'strb2', 7);
INSERT INTO dbo.Groups(groupid, memberid, string, val)
VALUES('b', 5, 'strb3', 3);
INSERT INTO dbo.Groups(groupid, memberid, string, val)
VALUES('b', 9, 'strb4', 11);
INSERT INTO dbo.Groups(groupid, memberid, string, val)
VALUES('c', 3, 'strc1', 8);
INSERT INTO dbo.Groups(groupid, memberid, string, val)
VALUES('c', 7, 'strc2', 10);
INSERT INTO dbo.Groups(groupid, memberid, string, val)
VALUES('c', 9, 'strc3', 12);
GO
USE tempdb;
GO
IF OBJECT_ID('dbo.Groups') IS NOT NULL
DROP TABLE dbo.Groups;
GO

CREATE TABLE dbo.Groups
(
groupid VARCHAR(10) NOT NULL,
memberid INT NOT NULL,
string VARCHAR(10) NOT NULL,
val INT NOT NULL,
PRIMARY KEY (groupid, memberid)
);

INSERT INTO dbo.Groups(groupid, memberid, string, val)
VALUES('a', 3, 'stra1', 6);
INSERT INTO dbo.Groups(groupid, memberid, string, val)
VALUES('a', 9, 'stra2', 7);
INSERT INTO dbo.Groups(groupid, memberid, string, val)
VALUES('b', 2, 'strb1', 3);
INSERT INTO dbo.Groups(groupid, memberid, string, val)
VALUES('b', 4, 'strb2', 7);
INSERT INTO dbo.Groups(groupid, memberid, string, val)
VALUES('b', 5, 'strb3', 3);
INSERT INTO dbo.Groups(groupid, memberid, string, val)
VALUES('b', 9, 'strb4', 11);
INSERT INTO dbo.Groups(groupid, memberid, string, val)
VALUES('c', 3, 'strc1', 8);
INSERT INTO dbo.Groups(groupid, memberid, string, val)
VALUES('c', 7, 'strc2', 10);
INSERT INTO dbo.Groups(groupid, memberid, string, val)
VALUES('c', 9, 'strc3', 12);
GO


--方法1：游标
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DECLARE
@Result TABLE(groupid VARCHAR(10), product BIGINT);
DECLARE
@groupid AS VARCHAR(10), @prvgroupid AS VARCHAR(10),
@val AS INT, @product AS BIGINT;

DECLARE C CURSOR FAST_FORWARD FOR
SELECT groupid,val FROM dbo.Groups ORDER BY groupid;

OPEN C

FETCH NEXT FROM C INTO @groupid, @val;
SELECT @prvgroupid = @groupid, @product = 1;

WHILE @@fetch_status = 0
BEGIN
IF @groupid <> @prvgroupid
BEGIN
INSERT INTO @Result VALUES(@prvgroupid, @product);
SELECT @prvgroupid = @groupid, @product = 1;
END

SET @product = @product * @val;

FETCH NEXT FROM C INTO @groupid, @val;
END

IF @prvgroupid IS NOT NULL
INSERT INTO @Result VALUES(@prvgroupid, @product);

CLOSE C;

DEALLOCATE C;

SELECT groupid, product FROM @Result;
GO
DECLARE
@Result TABLE(groupid VARCHAR(10), product BIGINT);
DECLARE
@groupid AS VARCHAR(10), @prvgroupid AS VARCHAR(10),
@val AS INT, @product AS BIGINT;

DECLARE C CURSOR FAST_FORWARD FOR
SELECT groupid,val FROM dbo.Groups ORDER BY groupid;

OPEN C

FETCH NEXT FROM C INTO @groupid, @val;
SELECT @prvgroupid = @groupid, @product = 1;

WHILE @@fetch_status = 0
BEGIN
IF @groupid <> @prvgroupid
BEGIN
INSERT INTO @Result VALUES(@prvgroupid, @product);
SELECT @prvgroupid = @groupid, @product = 1;
END

SET @product = @product * @val;

FETCH NEXT FROM C INTO @groupid, @val;
END

IF @prvgroupid IS NOT NULL
INSERT INTO @Result VALUES(@prvgroupid, @product);

CLOSE C;

DEALLOCATE C;

SELECT groupid, product FROM @Result;
GO

--方法2：PIVOTING 聚合

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SELECT groupid,
MAX(CASE WHEN rn = 1 THEN val ELSE 1 END)
* MAX(CASE WHEN rn = 2 THEN val ELSE 1 END)
* MAX(CASE WHEN rn = 3 THEN val ELSE 1 END)
* MAX(CASE WHEN rn = 4 THEN val ELSE 1 END) AS product
FROM (SELECT groupid, val,
(SELECT COUNT(*)
FROM dbo.Groups AS B
WHERE B.groupid = A.groupid
AND B.memberid <= A.memberid) AS rn
FROM dbo.Groups AS A) AS D
GROUP BY groupid;
--方法3：特殊方法
SELECT groupid, POWER(10., SUM(LOG10(val))) AS product
FROM dbo.Groups
GROUP BY groupid;
SELECT groupid,
MAX(CASE WHEN rn = 1 THEN val ELSE 1 END)
* MAX(CASE WHEN rn = 2 THEN val ELSE 1 END)
* MAX(CASE WHEN rn = 3 THEN val ELSE 1 END)
* MAX(CASE WHEN rn = 4 THEN val ELSE 1 END) AS product
FROM (SELECT groupid, val,
(SELECT COUNT(*)
FROM dbo.Groups AS B
WHERE B.groupid = A.groupid
AND B.memberid <= A.memberid) AS rn
FROM dbo.Groups AS A) AS D
GROUP BY groupid;
--方法3：特殊方法
SELECT groupid, POWER(10., SUM(LOG10(val))) AS product
FROM dbo.Groups
GROUP BY groupid;


------------结果----------------
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/*
groupid product
---------- ---------------------------------------
a 42
b 693
c 960

*/
/*
groupid product
---------- ---------------------------------------
a 42
b 693
c 960

*/


----上面三种方法，用游标虽然不是很快,但是它简单 通用 ,第二种方法需要组中的元素比较少,第三种方法奇快无比,但是它只适合用来算正数
---所以这里使用游标还是不错的.

b.连续聚合(累计组内从第一个元素到当前元素)

--插入数据

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IF OBJECT_ID('dbo.EmpOrders') IS NOT NULL
DROP TABLE dbo.EmpOrders;
GO

CREATE TABLE dbo.EmpOrders
(
empid INT NOT NULL,
ordmonth DATETIME NOT NULL,
qty INT NOT NULL,
PRIMARY KEY(empid, ordmonth)
);

INSERT INTO dbo.EmpOrders(empid, ordmonth, qty)
SELECT O.EmployeeID,
CAST(CONVERT(CHAR(6), O.OrderDate, 112) + '01'
AS DATETIME) AS ordmonth,
SUM(Quantity) AS qty
FROM Northwind.dbo.Orders AS O
JOIN Northwind.dbo.[Order Details] AS OD
ON O.OrderID = OD.OrderID
GROUP BY EmployeeID,
CAST(CONVERT(CHAR(6), O.OrderDate, 112) + '01'
AS DATETIME);
GO
set nocount on
IF OBJECT_ID('dbo.EmpOrders') IS NOT NULL
DROP TABLE dbo.EmpOrders;
GO

CREATE TABLE dbo.EmpOrders
(
empid INT NOT NULL,
ordmonth DATETIME NOT NULL,
qty INT NOT NULL,
PRIMARY KEY(empid, ordmonth)
);

INSERT INTO dbo.EmpOrders(empid, ordmonth, qty)
SELECT O.EmployeeID,
CAST(CONVERT(CHAR(6), O.OrderDate, 112) + '01'
AS DATETIME) AS ordmonth,
SUM(Quantity) AS qty
FROM Northwind.dbo.Orders AS O
JOIN Northwind.dbo.[Order Details] AS OD
ON O.OrderID = OD.OrderID
GROUP BY EmployeeID,
CAST(CONVERT(CHAR(6), O.OrderDate, 112) + '01'
AS DATETIME);
GO
set nocount on


--方法1：游标
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DECLARE @Result
TABLE(empid INT, ordmonth DATETIME, qty INT, runqty INT);
DECLARE
@empid AS INT,@prvempid AS INT, @ordmonth DATETIME,
@qty AS INT, @runqty AS INT;

DECLARE C CURSOR FAST_FORWARD FOR
SELECT empid, ordmonth, qty
FROM dbo.EmpOrders
ORDER BY empid, ordmonth;

OPEN C

FETCH NEXT FROM C INTO @empid, @ordmonth, @qty;
SELECT @prvempid = @empid, @runqty = 0;

WHILE @@fetch_status = 0
BEGIN
IF @empid <> @prvempid
SELECT @prvempid = @empid, @runqty = 0;

SET @runqty = @runqty + @qty;

INSERT INTO @Result VALUES(@empid, @ordmonth, @qty, @runqty);

FETCH NEXT FROM C INTO @empid, @ordmonth, @qty;
END

CLOSE C;

DEALLOCATE C;

SELECT empid, CONVERT(VARCHAR(7), ordmonth, 121) AS ordmonth,
qty, runqty
FROM @Result
ORDER BY empid, ordmonth;
GO
DECLARE @Result
TABLE(empid INT, ordmonth DATETIME, qty INT, runqty INT);
DECLARE
@empid AS INT,@prvempid AS INT, @ordmonth DATETIME,
@qty AS INT, @runqty AS INT;

DECLARE C CURSOR FAST_FORWARD FOR
SELECT empid, ordmonth, qty
FROM dbo.EmpOrders
ORDER BY empid, ordmonth;

OPEN C

FETCH NEXT FROM C INTO @empid, @ordmonth, @qty;
SELECT @prvempid = @empid, @runqty = 0;

WHILE @@fetch_status = 0
BEGIN
IF @empid <> @prvempid
SELECT @prvempid = @empid, @runqty = 0;

SET @runqty = @runqty + @qty;

INSERT INTO @Result VALUES(@empid, @ordmonth, @qty, @runqty);

FETCH NEXT FROM C INTO @empid, @ordmonth, @qty;
END

CLOSE C;

DEALLOCATE C;

SELECT empid, CONVERT(VARCHAR(7), ordmonth, 121) AS ordmonth,
qty, runqty
FROM @Result
ORDER BY empid, ordmonth;
GO


--方法2：集合的思想
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SELECT O1.empid, CONVERT(VARCHAR(7), O1.ordmonth, 121) AS ordmonth,
O1.qty AS qtythismonth,
(SELECT SUM(O2.qty)
FROM dbo.EmpOrders AS O2
WHERE O2.empid = O1.empid
AND O2.ordmonth <= O1.ordmonth) AS totalqty
FROM dbo.EmpOrders AS O1
GROUP BY O1.empid, O1.ordmonth, O1.qty;
SELECT O1.empid, CONVERT(VARCHAR(7), O1.ordmonth, 121) AS ordmonth,
O1.qty AS qtythismonth,
(SELECT SUM(O2.qty)
FROM dbo.EmpOrders AS O2
WHERE O2.empid = O1.empid
AND O2.ordmonth <= O1.ordmonth) AS totalqty
FROM dbo.EmpOrders AS O1
GROUP BY O1.empid, O1.ordmonth, O1.qty;


---基于游标的解决方案呢 只扫描表一次.方案性能成线性变化.基于集合的却是基于 N的平方 变化.
--一般如果有g组,一组里有N行,那么你将扫描 g*(n+n*n)/2行.这里假设列上有索引的情况下.如果你的组足够小,那么使用基于集合的方案
--要比游标快很多.随着组的增大,性能集合方案性能将以N*N的速度下降.而基于游标的解决方法性能是一条直线.
--从书上一个实验图看出结果:随着行数的增加,基于游标的方案性能的优势将越来越明显.因为一个是N*N的抛物线,一个是直线.

PS:当SQL支持 sum() over (partition by XX ORDER BY xxx)的时候 集合的这个方案可能比游标更加快

c.还有一些匹配方面的问题 游标也比较快.因为比较难,自己也没理解透彻.

说实话 我也不喜欢游标 但是它确实存在着 并且有它一定的用武之地目前.



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