Add or Remove IDENTITY Property From an Existing Column Efficiently

Introduction
Refactoring is often needed to improve schema design or address changes in requirements. Occasionally, these schema modifications require removing the IDENTITY property from an existing non-empty table while retaining the existing values. Unfortunately, there is no DDL command to add or remove IDENTITY from an existing column. Traditionally, one must go through the pain of either recreating the table or jumping through hoops by adding a new column, updating the new column value with the old column value, and dropping the original column. This is especially problematic with large tables and short maintenance windows.

This article details how to use SWITCH as an alternative technique to add or remove IDENTITY from an existing column. I have been using SWITCH for many years but learned only recently that the identical schema restriction for SWITCH does not include IDENTITY column property; one can SWITCH data from one table to another even though the IDENTITY column property differs. This behavior allows the IDENTITY property to be added or removed.

Overview
All tables are partitioned from a database storage engine perspective since SQL Server 2005, although multiple partitions require Enterprise Edition. The implication of this storage architecture is that one can use ALTER TABLE…SWITCH regardless of SQL Server edition to move the entire contents of one table to another without physically moving rows as long as the source and target tables have an identical schema and are physically aligned (indexes and tables on same filegroup). SWITCH performs fast storage meta-data changes so the operation typically takes less than a second regardless of table size. The target table must be empty before SWITCH. After SWITCH, the target table will contain the data and the source table will be empty (similar to TRUNCATE).

Since the identical schema restriction does not include the IDENTITY column property, SWITCH is a handy technique to add or remove IDENTITY from an existing column of a non-empty table. For an empty table, a simple drop and create is easier and more efficient.

Remove IDENTITY from an Existing Column
The script below uses a staging table without the IDENTITY column property along with SWITCH to remove the IDENTITY property from the column and retain previously assigned identity values. I drop the empty original table and rename the staging table to the original name along with constraints and indexes after the operation.

Add IDENTITY to an Existing Column
This script uses a staging table with the IDENTITY column property along with SWITCH to add the IDENTITY property while retaining the existing column values. After the SWITCH operation, DBCC CHECKIDENT seeds the next IDENTITY value greater than the current value in the column.

Note that an IDENTITY column must now allow NULL values so, in the case of a nullable column, the column must first be altered to NOT NULL before using this method.