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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: [email protected]• Blog: http://Sqlblogcasts.com/blogs/simons• Twitter: simon_sabin
Car crash
Works on my computer
Parameter Sniffing
@
SQL tries to be clever
A bit like
Statistics
So what are the root causes
Out of date statistics
Best Before: 1/4/1999
Skewed data
Multi purpose queries
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
Update Statistics
• Update Statistics– Can be a performance hit
• Trace flag 2388, 2389 and 2390
OverviewOut of date statistics
Compilation
Best Query
Skewed Data
Supportable
Multiple Code paths Update statistics OPTIMZE FOR WITH RECOMPILE Trace Flags
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
Q&A
• Now - Just ask• Afterwards – I’ll be around• Much Later– [email protected]– @simon_sabin– http://sqlblogcasts.com/blogs/simonsabin
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