SEQUENCE vs IDENTITY in SQL Server: Choosing the Right Auto-Increment
TL;DR Pick the wrong auto-increment strategy in SQL Server and you find out in production. An INT IDENTITY column hits 2,147,483,647. Replication nodes collide on the same keys. A failover jumps the counter by 10,000. IDENTITY for simple single-table surrogate keys. SEQUENCE for anything that crosses tables, needs pre-allocation, or has to survive a migration. Neither gives you gapless numbering.…
IDENTITY and SEQUENCE are two auto-increment strategies available in SQL Server for generating surrogate keys. While both produce gaps in numbering, they serve different purposes and have distinct characteristics.
IDENTITY is a column property that generates values at insert time. It offers two configuration options: seed and step. Once defined, IDENTITY cannot be modified without recreating the table. This makes it suitable for simple single-table surrogate keys where the requirements are straightforward and do not change over time.
On the other hand, SEQUENCE is a standalone object that provides more flexibility and options. It allows you to create a sequence independently and use it wherever needed. SEQUENCE offers six configurable options, including start value, increment by, minvalue, maxvalue, cycle, and cache. These options enable pre-allocation, range management across tables, replication node distribution, and cycle functionality.
CACHE is a performance option specific to SEQUENCE. It maintains a range of numbers in memory, reducing disk writes during bulk inserts. This can significantly improve performance, especially when dealing with high insert volumes.
IDENTITY, however, lacks this CACHE option and relies on disk writes for each row insertion. Additionally, IDENTITY does not support range management across tables, replication nodes, or cycle functionality, making it less versatile than SEQUENCE.
When choosing between IDENTITY and SEQUENCE, consider the following factors:
1. Scope: IDENTITY is table-bound and cannot be shared across multiple tables, while SEQUENCE operates at the database level and can be shared across multiple tables.
2. Pre-allocation: SEQUENCE allows pre-allocation of numbers, which is useful when sharing keys across tables or replicating data. IDENTITY does not support this.
3. Gap management: Neither IDENTITY nor SEQUENCE guarantees gapless numbering. Gaps can occur due to transaction rollbacks, server restarts, cache flushes, or explicit reseeding.
4. Performance: SEQUENCE with CACHE can significantly reduce disk writes during bulk inserts, improving performance in high-volume insert scenarios. IDENTITY relies on disk writes for each row insertion.
In summary, use IDENTITY for simple single-table surrogate keys with straightforward requirements. Opt for SEQUENCE when you need more flexibility, pre-allocation, range management across tables or replication nodes, or cycle functionality. Consider the performance implications of CACHE and the limitations of both mechanisms when making your decision.
Written by urgent.news from Dev.to's reporting — not their text. Machine-written; read the original for the full account.
