SQL: Will setting IDENTITY_INSERT ON disable updating the table's identity table?

Question

I'm currently working on a data migration project and for performance-related issues, I want to predefine a set of identities rather than letting the tables generate them.

I found it's not easy to add the identity property to a column, so I want to use IDENTITY_INSERT ON statement.

My question is: would this disable updates to the table's identity table (which is impacting performance), or do I need to truly remove the identity property of the column(s)?

Solution

It's very common for data migration scripts to have something like:

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

While enabled, the field will not auto-increment for other inserts.

IDENTITY_INSERT has session scope, so only your session will be able to insert to the identity row explicitly. AND only one table in a session can have IDENTITY_INSERT ON at a time.

So what about performance? I don't actually have an answer to you question, but I have some code that should give you an answer. It's a modified version of something I found here:

/* 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

I don't have a SQL SERVER handy to run this on, so let me know how it goes. Hope it helps.