What Happens When Multiple Sessions Call NEXT VALUE FOR SEQUENCE

When multiple sessions request sequence values simultaneously, SQL Server allocates unique increasing numbers safely and efficiently. Values are assigned immediately without waiting for commit, so gaps may occur. Order between sessions isn’t guaranteed, but duplication never happens. CACHE improves performance but may increase gaps.

SQL Server SEQUENCE guarantees that Every session gets a unique value No duplicates ever. Sequence values are allocated atomically. Allocation is thread-safe & highly concurrent SQL Server internally serializes sequence generation so parallel sessions never clash. Sequence does NOT wait for commit Values are assigned immediately. If a transaction rolls back then number is not returned. Order is not guaranteed across sessions Even though numbers are increasing, concurrency scheduling means that session 2 might receive a number before Session 1 commits.

When we enable CACHE then numbers are pre allocated and it will be Super-fast under concurrency but if SQL crashes, cached numbers can be lost (gaps). When we are not enabling CACHE its Strict persistence and slower than enable CACHE.

Let’s see the demo

Here creating a sequence

CREATE SEQUENCE Seq_Demo_HC AS BIGINT START WITH 1 INCREMENT BY 1 CACHE 50;

Now we are using this Sequence in multiple high transaction.

We have run this on several window.

--window 1 BEGIN TRAN; SELECT NEXT VALUE FOR Seq_Demo_HC AS Seq_Demo_HC; -- Do NOT commit yet

2^nd window

3^rd window

And so on.

All got unique IDs even before commit.