How Does Row Order Differ Between SQL Server 2008 and SQL Server 2012 SELECT Queries?

SQL Server 2008 vs. 2012: SELECT Query Row Order Discrepancies

The Problem:

Upgrading from SQL Server 2008 to SQL Server 2012 has revealed a critical difference: the default row order in SELECT queries without an ORDER BY clause. While SQL Server 2008 often returned rows in a seemingly consistent order, SQL Server 2012 provides no such guarantee.

The Impact:

This change significantly affects applications relying on this implicit ordering, particularly impacting over 2500 stored procedures across 5 databases. A blanket addition of ORDER BY clauses is impractical. Developers are seeking efficient solutions, even considering a downgrade to SQL Server 2008.

Why Explicit Ordering is Essential:

Downgrading is not recommended. The expectation of a default row order is fundamentally flawed. SQL operates on sets, and sets inherently lack inherent order. As Itzik Ben-Gan highlights in "Microsoft SQL Server 2012 T-SQL Fundamentals," query results are unordered unless explicitly sorted.

The Unpredictability of Implicit Order:

Developers must always use explicit ORDER BY clauses. The query optimizer might rearrange operations for performance, potentially altering any perceived default order.

Best Practices for Consistent Row Order:

To ensure predictable results:

• Always include ORDER BY in SELECT statements where row order is relevant.
• If the default order is critical, create an index enforcing that order.
• Note that even with ORDER BY, order might vary across different query plans, especially with parallel execution.

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