What does Microsoft guarantee if you upgrade versions of SQL Server while maintaining the same compatibility level?

When upgrading versions of SQL Server while maintaining the same compatibility level, Microsoft guarantees that the execution plan shape will remain consistent. This means that the way queries are optimized and the plans generated to execute those queries will not significantly change with the upgrade, provided the compatibility level is unchanged. This is crucial for database administrators as it helps ensure predictable performance and stability after an upgrade.

Maintaining the same execution plan shape allows system administrators to confidently upgrade without fearing drastic changes in query performance, as many are often concerned about execution plans being altered following an upgrade, which could lead to performance unpredictability. By guaranteeing the execution plan shape, Microsoft provides assurance to database administrators that they will get the same performance characteristics they have analyzed and optimized, ensuring a smoother transition to newer versions of SQL Server.