Mistake: ordering every query because ordered output feels safer. Sorting is a presentation operation, not a set-based requirement, and can make a query substantially slower. One comparison ran 25% slower with ordering enabled; its sort represented 91% of estimated plan cost. section 5.4
Compare unordered and ordered output
SET STATISTICS TIME ON; SELECT CustomerContactFirstName, CustomerContactLastName, CustomerContactEmail FROM dbo.CustomerContacts; SELECT CustomerContactFirstName, CustomerContactLastName, CustomerContactEmail FROM dbo.CustomerContacts ORDER BY CustomerContactEmail; Use Include Actual Execution Plan in SQL Server Management Studio when investigating the difference. Treat displayed costs as estimates rather than direct performance measurements. section 5.4
Apply these rules:
- Order rows only when the consumer genuinely requires a specific sequence. section 5.4
- Measure unordered and ordered versions on the same test setup, without capturing execution plans during timing tests. section 5.4
- If ordering is necessary, inspect whether a covering nonclustered index can improve the query. section 5.4
