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• 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Out of date statistics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– Simon@SQLKnowHow.com– @simon_sabin– http://sqlblogcasts.com/blogs/simonsabin

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