Sequence vs identity

Question

SQL Server 2012 introduced Sequence as a new feature, same as in Oracle and Postgres. Where sequences are preferred over identities? And why do we need sequence

Answer 1

Recently was bit by something to consider for identity vs sequence. Seems MSFT now suggests sequence if you may want to keep identity without gaps. We had an issue where there were huge gaps in the identity, but based on this statement highlighted would explain our issue that SQL cached the identity and after reboot we lost those numbers.

https://docs.microsoft.com/en-us/sql/t-sql/statements/create-table-transact-sql-identity-property?view=sql-server-2017

Consecutive values after server restart or other failures – SQL Server might cache identity values for performance reasons and some of the assigned values can be lost during a database failure or server restart. This can result in gaps in the identity value upon insert. If gaps are not acceptable then the application should use its own mechanism to generate key values. Using a sequence generator with the NOCACHE option can limit the gaps to transactions that are never committed.