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)?
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.