NEWSEQUENTIALID

NEWSEQUENTIALID is a Transact-SQL function in Microsoft SQL Server that generates a unique identifier (GUID) that is guaranteed to be sequential. This sequential nature makes it more efficient for SQL Server to store and retrieve data when used as a primary key or clustered index compared to the non-sequential GUID generated by the NEWID function. When NEWSEQUENTIALID is used for a primary key in a clustered index, it can help reduce page splits and improve insert performance, especially in scenarios with high insert volumes.

The function returns a uniqueidentifier data type. While it generates sequential values, it's important to note that the sequence is based on the time of generation and the machine's MAC address, so it's not a predictable or monotonically increasing sequence across different servers or restarts. However, within a single server instance, the GUIDs are generated in a predictable order, which benefits storage and indexing. NEWSEQUENTIALID cannot be used directly with the DEFAULT constraint on a column. Instead, it must be specified within a CREATE TABLE or ALTER TABLE statement. It is also not supported in distributed transactions.