How to prevent interim identity holes in SQL Server

Question

Is there a way (using config + transaction isolation levels) to ensure that there are no interim holes in a SQL Server IDENTITY column? Persistent holes are OK. The situation I am trying to avoid is when one query returns a hole but a subsequent similar query returns a row that was not yet committed when the query had been run the first time.

Answer 1

Your question is one of isolation levels and has nothing to do with IDENTITY. The same problem applies to any update/insert visibility. The first query can return results which had include an uncommited row in one and only one situation: if you use dirty reads (read uncommited). If you do, then you deserve all the inconsistent results you'll get and you deserve no help.

If you want to see stable results between two consecutive reads you must have a transaction that encompases both reads and use SERIALIZABLE isolation level or, better, use a row versioning based isolation level like SNAPSHOT. My recommendation would be to enable SNAPSHOT and use it. See Using Snapshot Isolation.

All I need is the promise that inserts to a table are committed in order of identity values they claim.

I hope you read this again and realize the impossibility of the request ('promise ... commit..'). You can't ask for something to guarantee success before it finished. What you're asking eventually boils down to asking not to allocate a new identity before the previous allocated one has committed successfully. In other words, full serialization of all insert transactions.