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SQL Performance 2011/12

SQL Performance 2011/12. Joe Chang, SolidQ www.qdpma.com http:// sqlblog.com/blogs/joe_chang/default.aspx jchang6@yahoo.com. 2011. Hardware is Powerful & Cheap CPU (cores), memory, and now IO too! Quad-core since 2006, 1GHz since 2000 Is Performance still a concern?

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SQL Performance 2011/12

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  1. SQL Performance 2011/12 Joe Chang, SolidQwww.qdpma.com http://sqlblog.com/blogs/joe_chang/default.aspx jchang6@yahoo.com

  2. 2011 • Hardware is Powerful & Cheap • CPU (cores), memory, and now IO too! • Quad-core since 2006, 1GHz since 2000 • Is Performance still a concern? • Yes, along with fundamentals • Modern Performance Strategy • Can handle minor inefficiencies • Identify and circumvent the really bad things

  3. Modern Hardware • 4-12 cores per processor socket • 16GB DIMM at less than $1K • SSD • Enterprise grade SSD still moderately expensive • Both SLC and MLC • Consumer SSD – really cheap, $3-4K per TB • Uneven performance characteristics over time • Desired IO performance: 1-2GB/s, 20-50K IOPS

  4. Hardware Baseline 2011 Entry Mid-range 2 Xeon 5600 6-core 48 – 192GB 6 x 8GB to 12 x 16GB SSD options 16+ SATA SSDs 4-5 PCI-E SSDs • 1 Xeon E3 quad-core • 16GB memory • 4 x 4GB unbuffered ECC • SSD options: • 2-4 SATA SSDs • 1-2 PCI-E SSDs

  5. Performance Fundamentals • Network round-trips • Owner qualified, case correct • Log write latencies • Sufficiently low to support transaction volume • Not necessarily separate data and log disks • Normalization – correct data trumps all! • Indexes – a few good ones, and not too many! • SQL – that the optimizer

  6. What can go wrong? • With immense hardware resources • And a great database engine • What can go wrong? • Following a fixed set of rules and procedures • Basic transactions processing should work well • If your process does something unanticipated • Some things can go horribly wrong

  7. Performance Concepts • Query Optimizer • Execution plan operators • Formula for component operation • Data distribution statistics • to estimate rows & pages, automatically updated • Rules when estimate not possible • Stored procedure compile rules • Parameters and variables

  8. Stored Procedure Basics

  9. Parameters and Variables • On compile • Parameter values used to for row estimate • Variables – assume unknown value • Consider effect of skewed distribution

  10. Parameter & Variable 6 rows for value 1 4 rows for value unknown Consider impact for skewed data distributions

  11. Stored Procedure Compile Options • WITH RECOMPILE • OPTIMIZE FOR • Plan Guide • Temp table • KEEP PLAN, KEEPFIXED PLAN

  12. Compile & Execute Time • Plan reuse desired when • Compile cost is high relative to execute cost • Recompile desired when • Execute cost is high relative to compile cost

  13. Statistics Basics

  14. Statistics • No statistics – table variables • Temp table – statistics auto recompute • 6 row modified, 500 rows, every 20% thereafter • Statistics sampling • Random page, how to handle skewed distribution? • Upper and lower bounds • Problems caused by incrementing columns • Propagation errors

  15. Statistics Recompute • Scenario: start with accurate statistics • Update column with new values • That did not previously exist • If fewer than 20% of rows updated • Auto-recompute is not triggered

  16. Sampling • Default sampling percentage is usually good • Caution: not a random row sample! • Random sampling of page • From nonclustered index if available • If there is correlation between pages & values • Then serious over estimation possible

  17. Out of range • Statistics sampling tries to identify lower and upper bound

  18. Bad Execution Plan Examples Not comprehensive

  19. Scenarios Not comprehensive • Or condition • Multiple optional search arguments • Skewed distributions • 1 business logic for Small and large data sets • Reports for 1 day, 1 week, 1 month, 1 year • Statistics related problems • Resulting in horrible execution plan

  20. When the Query Optimizer Does not understand you

  21. Simple OR Conditions

  22. Simple Index Seek

  23. OR Condition in Join

  24. Alternative – Union

  25. UNION and UNION ALL • UNION • Only distinct rows • Sort to eliminate duplicates • Can be expensive for high row counts • UNION ALL • All rows • No sort to eliminate duplicates

  26. Multiple Optional SARGs This was suppose to work, but does not

  27. Parameterized SQL

  28. Skew and Range Variation

  29. Statistics Out-of Range

  30. Statistics Out of Range (cont.)

  31. Table Variable No Statisticsassumes 1 row, 1page Why? No recompiles

  32. Loop Join – Scan Inner Source Estimate 1 row Really Bad News

  33. The Correct Plan Estimate 1 row Hash Join forcedwith hint

  34. Loop Join vs. Hash Join

  35. Alternative Plan with Index

  36. Temp Table versus CTE • Consider options • SELECT xxx INTO #Temp • FROM Sql Main Expression • WITH tmp AS (SELECT xxx FROM Sql) Main Expression

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