Question

我目前正致力于数据迁移项目和与性能相关的问题，我想预定义一组身份，而不是让表自己生成它们。

我发现将identity属性添加到列并不容易，因此我想使用IDENTITY_INSERT ON语句。

我的问题是：这是否会禁用对表的标识表的更新（这会影响性能），还是我需要真正删除列的identity属性？

Answer 1

数据迁移脚本通常具有以下内容：

SET IDENTITY_INSERT [MyTable] ON
INSERT INTO [MyTable] ...
INSERT INTO [MyTable] ...
INSERT INTO [MyTable] ...
...
SET IDENTITY_INSERT [MyTable] OFF

启用后，该字段不会自动增加其他插入。

IDENTITY_INSERT具有会话范围，因此只有您的会话才能显式插入标识行。并且会话中只有一个表可以一次打开IDENTITY_INSERT。

那么性能呢？我实际上没有回答你的问题，但我有一些代码可以给你一个答案。这是我发现的here：

的修改版本

/* Create a table with an identity value */
CREATE TABLE test_table
  (
     auto_id  INT IDENTITY(1, 1),
     somedata VARCHAR(50)
  )
GO 

/* Insert 10 sample rows */
INSERT INTO test_table
SELECT 'x'
GO 10

/* Get the current identity value (10) */
SELECT Ident_current('test_table') AS IdentityValueAfterTenInserts

GO

/* Disable the identity column, insert a row, enable the identity column. */
SET identity_insert test_table ON
INSERT INTO test_table(auto_id, somedata)
SELECT 50, 'x'
SET identity_insert test_table OFF 

GO

/* Get the current identity value (50) */
SELECT Ident_current('test_table') AS IdentityValueAfterIdentityInsertWithIdentityEnabled

GO

/* Disable the identity column, insert a row, check the value, then enable the identity column. */
SET identity_insert test_table ON
INSERT INTO test_table(auto_id, somedata)
SELECT 100, 'x'

/* 
   Get the current identity value (?) 
   If the value is 50, then the identity column is only recalculated when a call is made to:
       SET identity_insert test_table OFF
   Else if the value is 100, then the identity column is recalculated constantly and your 
   performance problems remain.
*/
SELECT Ident_current('test_table') AS IdentityValueAfterIdentityInsertWithIdentityDisabled


SET identity_insert test_table OFF 

GO
/* Get the current identity value (100) */
SELECT Ident_current('test_table') AS IdentityValueAfterIdentityInsertWithIdentityEnabled

GO

DROP TABLE test_table

我没有方便运行它的SQL SERVER，所以让我知道它是怎么回事。希望它有所帮助。

Answer 2

关于带有 SET IDENTITY_INSERT ON 的 Identity 列的注意事项。

刚刚检查了 SQL 2012，如果您打开该选项，则无法使用内置的自动标识插入。

下面的快速测试...

BEGIN TRY DROP TABLE #T END TRY BEGIN CATCH END CATCH;

CREATE TABLE #T (id int IDENTITY, Name varchar(50));

INSERT INTO #T (Name) VALUES ('Darren'); -- built in Identity format

SET IDENTITY_INSERT #T ON;

INSERT INTO #T (id, Name) VALUES (5, 'Jerry'); -- explicit format of identity

INSERT INTO #T (Name) VALUES ('Thomas');  -- TRY to use built in format

SET IDENTITY_INSERT #T OFF;

SELECT * FROM #T;

结果...

(1 row(s) affected)

(1 row(s) affected)
Msg 545, Level 16, State 1, Line 11
Explicit value must be specified for identity column in table '#T__________________________________________________________________________________________________________________000000C34998' either when IDENTITY_INSERT is set to ON or when a replication user is inserting into a NOT FOR REPLICATION identity column.

(2 row(s) affected)

SQL：设置IDENTITY_INSERT ON会禁用更新表的标识表吗？

2 个答案: