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Ten query tuning techniques every SQL Server programmer should know


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From the noted database expert and author of 'SQL in a Nutshell' - SELECT statements have a reputation for being very easy to write, but hard to write very well. This session will take you through ten of the most problematic patterns and anti-patterns when writing queries and how to deal with them all. Loaded with live demonstrations and useful techniques, this session will teach you how to take your SQL Server queries mundane to masterful.

Published in: Data & Analytics
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Ten query tuning techniques every SQL Server programmer should know

  1. 1. TEN QUERY TUNING TECHNIQUES Every SQL Programmer Should Know Kevin Kline Director of Engineering Services at SQL Sentry Microsoft MVP since 2003 Facebook, LinkedIn, Twitter at KEKLINE,
  2. 2. FOR FRIENDS OF SQL SENTRY • Free Plan Explorer download: • Free query tuning consultations: • Free new ebook (regularly $10) to attendees. Send request to • SQL Server educational videos, scripts, and slides: http://SQLSentry.TV • Tuning blog: • Monthly eNews tips and tricks: asp
  3. 3. AGENDA • Introductions • Test & tuning environment • 1. Clearing caches • Looking for red flags • 2. Reading execution plans • Query tuning techniques: • 8 more specific examples of widespread approaches that lead to poor performance • Summary 3
  4. 4. TEST & TUNING ENVIRONMENT • Code to clear the caches*: o CHECKPOINT o DBCC [FreeProcCache | FreeSystemCache | FlushProcInDB(<dbid>) ] o DBCC DropCleanBuffers • Code to set measurements: o SET STATISTICS [TIME | IO] o SET SHOWPLAN [TEXT | XML] or Graphic Execution Plans • Code for Dynamic Management Views (DMV) checks. o System info – sys.dm_os_performance_counters, sys.dm_os_wait_stats o Query info – sys.dm_exec_requests o Index info – sys.dm_db_index_usage_stats, sys.dm_io_virtual_file_stats
  5. 5. RED FLAGS IN YOUR SQL CODE • Red Flags Query Operators: o Lookups, Scans, Spools, Parallelism Operations • Other Red Flags: o Dissimilar estimated versus actual row counts o High physical reads o Missing statistics alarms o Large sort operations o Implicit data type conversions
  6. 6. DEMOS: DEFAULT CURSORS • I don’t always use cursors… o …but when I do, I avoid the default options o Slow and heavy-handed: Global, updateable, dynamic, scrollable o I use LOCAL FAST_FORWARD o May want to test STATIC vs. DYNAMIC, when tempdb is a bottleneck • Blog post:
  7. 7. DEMOS: WHERE IN VERSUS WHERE EXISTS • There are lots of ways to find data existing within subsets: • IN, EXISTS, JOIN, Apply, subquery • Which technique is best? • Blog post:
  8. 8. OPTIMIZING FOR SELECT VERSUS DML • Big differences between a SELECT and a DML statement that effects the same rows. • Shouldn’t blindly create every index the Tuning Advisor or execution plan tells you to! • Blog post -
  9. 9. READS & INDEX STRUCTURE • 8K pages • Leaf pages ARE the data. • Non-leaf pages are pointers. Leaf Pages Root Page Level 0 Intermediate Pages Level 1 Level 2
  10. 10. WRITES & INDEX STRUCTURE • Each change to the leaf pages requires all index structures be updated. Leaf Pages Root Page Level 0 Intermediate Pages Level 1 Level 2 Page Split DML Actual place-ment
  11. 11. DEMOS: UNWANTED RECOMPILES Execution In Memory? NO Load metadata compile optimize Execute YES ReComp
  12. 12. CAUSES OF RECOMPILE • Expected: Because we request it: • CREATE PROC … WITH RECOMPILE or EXEC myproc … WITH RECOMPILE • SP_RECOMPILE foo • Expected: Plan was aged out of memory • Unexpected: Interleaved DDL and DML • Unexpected: Big changes since last execution: • Schema changes to objects in underlying code • New/updated index statistics • Sp_configure
  13. 13. INTERLEAVED DDL AND DML • CREATE PROC testddldml AS … ; • CREATE TABLE #testdml; -- (DDL) • <some T-SQL code here> • INSERT INTO #testdml; -- (DML + RECOMPILE) • <some T-SQL code here> • ALTER TABLE #testdml; -- (DDL) • <some T-SQL code here> • INSERT INTO #testdml; -- (DML + RECOMPILE) • <some T-SQL code here> • DROP TABLE #testdml; -- (DDL) • <some T-SQL code here>
  14. 14. DEMOS: THE "KITCHEN SINK" PROCEDURE • Usually see it as a one-query-for-all-queries procedure, or even one-proc-for-for-all-transactions procedure: o Where name starts with S, or placed an order this year, or lives in Texas o Insert AND Update AND Delete AND Select • Conflicting optional parameters make optimization impossible o OPTION (RECOMPILE) o Dynamic SQL + Optimize for ad hoc workloads o Specialized procedures • Better approach? o Specialize and optimize each piece of code to do ONE THING really effectively
  15. 15. DEMOS: SP_EXECUTESQL VS. EXEC(…) • I don’t always use dynamic SQL… o …but when I do, I always use sp_executesql o Less fuss with concatenation and implicit/explicit conversions o Better protection against SQL injection (but not for all things) o At worst case, behavior is the same • Can promote better plan re-use • Encourages strongly typed parameters instead of building up a massive string
  16. 16. IMPLICIT CONVERSIONS • SQL Server has to do a lot of extra work / scans when conversion operations are assumed by the SQL programmer. • Happens all the time with data types you’d think wouldn’t need it, e.g. between date types and character types. • Very useful data type conversion chart at • Data type precedence call also have an impact:
  17. 17. IMPLICIT CONVERSION RESOURCES • Ian Stirk’s Column Mismatch Utility at 5138/. • Jonathan Kehayias’ plan cache analyzer at /01/08/finding-implicit-column-conversions-in-the-plan-cache. aspx. • Jonathan Kehayias’ index scan study at implicit-conversion-costs
  18. 18. DEMOS: COMMA-DELIMITED PARAMETERS • Example: pass a comma-separated list of OrderIDs • String splitting is expensive, even using CLR • Table-valued parameters are typically a better approach
  19. 19. DEMOS: TEMPORARY STRUCTURES • Which are better, temp tables or temp variables? Temp Table Temp Variable Stored in? Tempdb Tempdb Statistics? Yes No (1 row) Indexs/Keys? Yes 1 UK / PK only Truncate? Yes No Recompiles? Yes No Parallelism? Yes No Metadata Overhead? Low Lowest Lock Overhead? Normal Lowest
  20. 20. CODING STANDARDS AND DISSIMILARITY • Might sound frivolous, but naming schemes are important o Convention is not important; but rather being consistent and logical • Story: dbo.UpdateCustomer vs. dbo.Customer_Update • Always specify schema when creating, altering, referencing objects o Object resolution works a little bit harder without it o More importantly, it can get the wrong answer o And will often yield multiple copies of the same plan • Do not use the sp_ prefix on stored procedures o This has observable overhead, no matter how specific you are
  21. 21. MIMICKING PRODUCTION • Your dev machine is usually nothing like production o Build representative data when you can o Build a stats-only database when you can’t (a.k.a. a database clone) • Will allow you to see plan issues, but not speed o Make sure settings are the same • @@VERSION, edition • Max memory if possible, sp_configure options • Logins (and permissions), tempdb settings • Parameterization settings, recovery model, compression, snapshot isolation • Compatibility level (usually not an issue when working with a restore) • Run a full business cycle workload after a restore o Simulate equivalent hardware: DBCC OPTIMIZER_WHATIF o Use Distributed Replay when you can • Not perfect, but more realistic than single-threaded trace replay
  22. 22. SUMMARY Let’s connect! Facebook, LinkedIn, Twitter at KEKLINE. Email at Blogs at And
  23. 23. WRAP UP Engage with us on social media o I’m thankful for your word of mouth promotions and endorsements! Share your tough SQL tuning problems with us: Download SQL Sentry Plan Explorer for free: Check out our other award winning tools:
  24. 24. NOLOCK
  25. 25. NOLOCK • It is a turbo button …if you’re ok with inaccuracy • There are times it is perfectly valid o Ballpark row counts o Please use session-level setting, not table hint • Usually, though, better to use SNAPSHOT or RCSI o But test under heavy load