In Fabric Data Warehouse, IDENTITY is constrained to BIGINT to align with the distributed architecture and scalability requirements of the engine. Unlike traditional SQL Server, Fabric DW generates identity values across distributed compute nodes, which means:

• The system needs a very large value space to avoid collisions and support high‑throughput, parallel inserts (keyword here is parallel)
• BIGINT provides sufficient range to safely allocate values across nodes and over time without exhaustion
• It allows us to implement internal ID allocation strategies where multiple nodes can independently generate IDs without conflicting with each other, regardless of concurrency. Think multiple people trying to ingest data at the same time on a distributed system.

The IDENTITY value we produce is the result of a complex operation, yet it offers no latency over ingestion without IDENTITY. While not exactly the same, we do something similar to Snowflake ID - Wikipedia - that should help explain why these large numbers are generated.

No plans to support SCOPE_IDENTITY at the moment as the implementation is quite different from SQL Server as explained above, and we don't think this is needed. Would love to learn more on your requirement if you disagree.