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When query plans go wrong

When query plans go wrong. SQL. Simon Sabin. Independent SQL Server Consultant and Trainer Database design and development, Business Intelligence, Performance tuning and troubleshooting SQL Server since 6.5 Email: Simon@onarc.com Blog: http://Sqlblogcasts.com/blogs/simons

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When query plans go wrong

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  1. When query plans go wrong

  2. SQL

  3. Simon Sabin • Independent SQL Server Consultant and Trainer • Database design and development, Business Intelligence, Performance tuning and troubleshooting • SQL Server since 6.5 • Email: Simon@onarc.com • Blog: http://Sqlblogcasts.com/blogs/simons • Twitter: simon_sabin

  4. Car crash

  5. Works on my computer

  6. Parameter Sniffing @

  7. SQL tries to be clever

  8. A bit like

  9. Statistics

  10. So what are the root causes

  11. Out of date statistics Best Before: 1/4/1999

  12. Skewed data

  13. Multi purpose queries

  14. Solutions • Selective code paths • Careful as SP is compiled as one batch • WITH RECOMPILE • Compilation hit, plan cache bloat • OPTIMIZE FOR • Results in a consistent plan • PLAN guides • Results in a consistent plan

  15. Update Statistics • Update Statistics • Can be a performance hit • Trace flag 2388, 2389 and 2390

  16. Overview

  17. Summary • You will only know if you monitor • Baseline your system • Identify changes in read, writes and cpu • Not duration • Consider the options for your situation • You can win this battle

  18. Q&A • Now - Just ask • Afterwards – I’ll be around • Much Later • Simon@SQLKnowHow.com • @simon_sabin • http://sqlblogcasts.com/blogs/simonsabin Please fill in feedback forms

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