有时候，我们在某一重要的时间段需要监控某张表的变化情况，包含插入、更新、删除。举例来说，当我们把数据导出到外部的系统时，我们希望导出的是全部的数据，而且最好是导出上次导出之后变动的数据。
作为DBA，我们可采传统的触发器操作，来构建一个元数据表或一个时间戳列来监控数据的变化。
代码如下：Code Listing 1
该代码在 SQL 2005(SP3), SQL 2008 R2 (RTM with cu5)测试通过
-------------------
--Method 1: TRIGGER
-------------------
--Base Table Definition
IF OBJECT_ID('CheckSumTest', 'U') IS NOT NULL DROP TABLE CheckSumTest
GO
CREATE TABLE CheckSumTest
(
id int IDENTITY(1,1) NOT NULL PRIMARY KEY,
vc1 varchar(1) NOT NULL,
vc2 varchar(1) NOT NULL
)
GO
INSERT dbo.CheckSumTest (vc1, vc2) SELECT 'a', 'b'
INSERT dbo.CheckSumTest (vc1, vc2) SELECT 'b', 'a'
GO
--Create Audit Summary Table to hold Meta-Data
IF OBJECT_ID('dbo.TableAuditSummary', 'U') IS NOT NULL DROP TABLE dbo.TableAuditSummary
CREATE TABLE dbo.TableAuditSummary
( id INT IDENTITY(1,1) NOT NULL PRIMARY KEY,
TableName sysname NOT NULL,
LastUpdate DATETIME NOT NULL,
LastExport DATETIME NOT NULL
)
GO
INSERT dbo.TableAuditSummary (TableName, LastUpdate, LastExport) VALUES ('dbo.CheckSumTest', GETDATE(), GETDATE())
GO
--Tables that need exporting
SELECT * FROM dbo.TableAuditSummary WHERE LastUpdate>LastExport
--Create Trigger on all Base Tables
--This fires on any insert/update/delete and writes new LastUpdate column for the table set to Current Date and Time
IF OBJECT_ID('dbo.trg_CheckSumTest_MaintainAuditSummary', 'TR') IS NOT NULL DROP TRIGGER dbo.trg_CheckSumTest_MaintainAuditSummary
GO
CREATE TRIGGER dbo.trg_CheckSumTest_MaintainAuditSummary
ON dbo.CheckSumTest
AFTER INSERT, UPDATE, DELETE
AS
BEGIN
IF (object_id('dbo.CheckSumTest') IS NOT NULL)
UPDATE dbo.TableAuditSummary SET LastUpdate=GETDATE() WHERE TableName='dbo.CheckSumTest'
END
GO
--Make an Update
UPDATE dbo.CheckSumTest SET vc1='b', vc2='a' WHERE id=1
UPDATE dbo.CheckSumTest SET vc1='a', vc2='b' WHERE id=2
--Check Meta-Data
SELECT * FROM dbo.TableAuditSummary WHERE LastUpdate>LastExport
--When we have Exported the data, we run the following to reset MetaData
UPDATE dbo.TableAuditSummary SET LastExport=GETDATE() WHERE LastUpdate>LastExport
最近我正在读关天SQLSERVER在线帮助（BOL）相关的知识, 我接触到了 SQL Server CHECKSUM(), BINARY_CHECKSUM(), and CHECKSUM_AGG() 这几个函数, 由此突然想到这些函数是不是也可以监控表的数据变化，而事实证明CHECKSUM_AGG() 函数尽管被描述为检测表的变化，但这里不适用.
使用 CheckSum() and CheckSum_Agg() 函数
CHECKSUM_AGG() 函数, 在Books OnLine 和许多相关的站点上是这样描述的, 通常用于检测一个表的数据是否更改. 这是一个代替触发器的更好的方法，只是该操作会引起表扫描的操作。于是我这次我仍然使用元数据来跟踪数据的变化，只是新建了列LastChkSum代替了LastUpdate,该列用于保存CHECKSUM_AGG(BINARY_CHECKSUM(*)),它将会在全表中产生一个唯一值，以区别数据的变化情况。
代码如下： Listing 2.
---------------------------------------------
--Method 2 : using CheckSum (not reliable)
---------------------------------------------
--Base Table Definition
IF OBJECT_ID('CheckSumTest', 'U') IS NOT NULL DROP TABLE CheckSumTest
GO
CREATE TABLE CheckSumTest
(
id int IDENTITY(1,1) NOT NULL PRIMARY KEY,
vc1 varchar(1) NOT NULL,
vc2 varchar(1) NOT NULL
)
GO
INSERT dbo.CheckSumTest (vc1, vc2) SELECT 'a', 'b'
INSERT dbo.CheckSumTest (vc1, vc2) SELECT 'b', 'a'
GO
--Create Audit Summary Table to hold Meta-Data
IF OBJECT_ID('dbo.TableAuditSummary', 'U') IS NOT NULL DROP TABLE dbo.TableAuditSummary
CREATE TABLE dbo.TableAuditSummary
( id INT IDENTITY(1,1) NOT NULL PRIMARY KEY,
TableName sysname NOT NULL,
LastChkSum INT NOT NULL
)
GO
INSERT dbo.TableAuditSummary (TableName, LastChkSum)
SELECT 'dbo.CheckSumTest', CHECKSUM_AGG(BINARY_CHECKSUM(*)) FROM dbo.CheckSumTest
GO
--Tables that need exporting
SELECT * FROM dbo.TableAuditSummary WHERE TableName='dbo.CheckSumTest'
AND LastChkSum<>(SELECT CHECKSUM_AGG(BINARY_CHECKSUM(*)) FROM dbo.CheckSumTest)
UNION ALL
...
--Make a Simple (Single row) Update
UPDATE dbo.CheckSumTest SET vc1='c', vc2='a' WHERE id=1
--Tables that need exporting
SELECT * FROM dbo.TableAuditSummary WHERE TableName='dbo.CheckSumTest'
AND LastChkSum<>(SELECT CHECKSUM_AGG(BINARY_CHECKSUM(*)) FROM dbo.CheckSumTest)
UNION ALL
...
--Reset MetaData
UPDATE dbo.TableAuditSummary SET LastChkSum=(SELECT CHECKSUM_AGG(BINARY_CHECKSUM(*)) FROM dbo.CheckSumTest)
WHERE TableName='dbo.CheckSumTest'
--Make a Symmetric change
UPDATE dbo.CheckSumTest SET vc1='b', vc2='a' WHERE id=1
UPDATE dbo.CheckSumTest SET vc1='c', vc2='a' WHERE id=2
--Tables that need exporting (no rows returned as CHECKSUM_AGG() has not changed!!)
SELECT * FROM dbo.TableAuditSummary WHERE TableName='dbo.CheckSumTest'
AND LastChkSum<>(SELECT CHECKSUM_AGG(BINARY_CHECKSUM(*)) FROM dbo.CheckSumTest)
UNION ALLCode Listing 2
正如你所看到的那样，对于单个的变化的情况，CHECKSUM是使用比较好的，但是CHECKSUM_AGG()却不能反应数据的变化
代码如下：Code Listing 3
--Base Table Definition
IF OBJECT_ID('CheckSumTest', 'U') IS NOT NULL DROP TABLE CheckSumTest
GO
CREATE TABLE CheckSumTest
(
id int IDENTITY(1,1) NOT NULL PRIMARY KEY,
vc1 varchar(1) NOT NULL,
vc2 varchar(1) NOT NULL,
chksum1 AS (CHECKSUM(id, vc1, vc2)),
chksum2 AS (BINARY_CHECKSUM(id, vc1, vc2))
)
GO
INSERT dbo.CheckSumTest (vc1, vc2) SELECT 'a', 'b'
INSERT dbo.CheckSumTest (vc1, vc2) SELECT 'b', 'a'
GO
--Show Computed Columns and CheckSum_Agg() value = 199555
SELECT * FROM CheckSumTest
SELECT CHECKSUM_AGG(BINARY_CHECKSUM(*)) FROM CheckSumTest
--Make a Simple (Single row) Update
UPDATE dbo.CheckSumTest SET vc1='c', vc2='a' WHERE id=1
--Show Computed Columns and CheckSum_Agg() value = 204816 (Ok)
SELECT * FROM CheckSumTest
SELECT CHECKSUM_AGG(BINARY_CHECKSUM(*)) FROM CheckSumTest
--Make a Symmetric change
UPDATE dbo.CheckSumTest SET vc1='b', vc2='a' WHERE id=1
UPDATE dbo.CheckSumTest SET vc1='c', vc2='a' WHERE id=2
--Show Computed Columns and CheckSum_Agg() value = 204816 (Not Ok!)
SELECT * FROM CheckSumTest
SELECT CHECKSUM_AGG(BINARY_CHECKSUM(*)) FROM CheckSumTest
我们会发现调整前后 CHECKSUM_AGG(BINARY_CHECKSUM(*)) 的值是一样的，不能区分
结论：
CHECKSUM_AGG() 函数尽管被描述为能监测表数据的变化，在实际测试中是不行的。尤其是对表进行对称数据修改时，无法监测
作者：Tyler Ning
