How Does the Default Row Order Change in SQL Server 2012 Affect SELECT Queries?

Understanding the Row Order Shift in SQL Server 2012

Upgrading to SQL Server 2012 introduces a key change: the default row order returned by SELECT queries without an ORDER BY clause is no longer predictable. SQL Server 2012's adherence to set theory means row order is undefined unless explicitly specified.

Why the Row Order Change?

While it might seem SQL Server 2008 guaranteed row order without ORDER BY, this wasn't actually the case. The apparent order was often a consequence of underlying factors like indexes and physical data storage. Hardware changes or index rebuilds post-upgrade could drastically alter the execution plan and, consequently, the row order.

Solutions to Maintain Consistent Row Order

The most effective solution is to proactively address this:

• Implement ORDER BY Clauses: The recommended approach is to add ORDER BY clauses to all relevant stored procedures and queries. This guarantees consistent row ordering regardless of database environment or underlying changes.
• Reverting to SQL Server 2008: While a potential option, downgrading is complex, involving data re-import, stored procedure updates, and the risk of introducing new compatibility problems.

Key Takeaway:

The absence of an ORDER BY clause has never implicitly defined row order in SQL Server. Relying on an assumed order can cause unpredictable behavior and performance problems. Always explicitly specify the desired row order using ORDER BY for reliable and consistent results.

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