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POPerformance Tuning SQL - How?
 

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    <p><strong>How does one performance tune a SQL Query?</strong></p> <ul> <li>What tricks/tools/concepts can be used to change the performance of a SQL Query?</li> <li>How can the benefits be Quantified?</li> <li>What does one need to be careful of?</li> </ul> <p><br> <strong>What tricks/tools/concepts can be used to change the performance of a SQL Query?</strong></p> <ul> <li>Using Indexes? How do they work in practice?</li> <li>Normalised vs Denormalised Data? What are the performance vs design/maintenance trade offs?</li> <li>Pre-processed intermediate tables? Created with triggers or batch jobs?</li> <li>Restructure the query to use Temp Tables, Sub Queries, etc?</li> <li>Separate complex queries into multiples and UNION the results?</li> <li>Anything else?</li> </ul> <p><br> <strong>How can performance be Quantified?</strong></p> <ul> <li>Reads?</li> <li>CPU Time?</li> <li>"% Query Cost" when different versions run together?</li> <li>Anything else?</li> </ul> <p><br> <strong>What does one need to be careful of?</strong></p> <ul> <li>Time to generate Execution Plans? (Stored Procs vs Inline Queries)</li> <li>Stored Procs being forced to recompile</li> <li>Testing on small data sets (Do the queries scale linearly, or square law, etc?)</li> <li>Results of previous runs being cached</li> <li>Optimising "normal case", but harming "worst case"</li> <li>What is "Parameter Sniffing"?</li> <li>Anything else?</li> </ul> <p><br> <em>Note to moderators:</em> This is a huge question, should I have split it up in to multiple questions?</p> <p><em>Note To Responders:</em> Because this is a huge question please reference other questions/answers/articles rather than writing lengthy explanations.</p>
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