A query hint is a T-SQL directive that influences or overrides the SQL Server Query Optimizer. Use one only when the optimizer produces a demonstrably suboptimal plan; otherwise, work with the optimizer rather than against it. section 10.5
Mistake: forcing a join too quickly
A query joining Marketing.marketing.Impressions with MarketingArchive.dbo.ImpressionsArchive took 19 seconds because the plan used Nested Loops. Forcing a Hash Join changed the selected physical operator, but the safer first investigation is whether indexes on the join columns can improve the plan. Query hints can resolve exceptional performance problems, yet they may work against better decisions for different data or conditions. section 10.5
