Ten query tuning techniques every SQL Server programmer should know
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The document outlines ten essential SQL query tuning techniques that every programmer should be aware of, emphasizing the importance of understanding execution plans, caching, and optimization strategies. It includes practical advice on identifying performance red flags, utilizing appropriate SQL commands, and improving code efficiency through various coding practices. Additionally, the document addresses potential pitfalls and best practices for managing queries and database structures effectively.
![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](https://image.slidesharecdn.com/ug-tenquerytuningtechniqueseverysqlprogrammershouldknow-140828230619-phpapp02/85/Ten-query-tuning-techniques-every-SQL-Server-programmer-should-know-4-320.jpg)
Ten query tuning techniques every SQL Server programmer should know
- 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 [email protected] KevinEKline.com, ForITPros.com
- 2. FOR FRIENDS OF SQL SENTRY • Free Plan Explorer download: http://www.sqlsentry.net/plan-explorer/ • Free query tuning consultations: http://answers.sqlperformance.com. • Free new ebook (regularly $10) to attendees. Send request to [email protected]. • SQL Server educational videos, scripts, and slides: http://SQLSentry.TV • Tuning blog: http://www.sqlperformance.com/ • Monthly eNews tips and tricks: http://www.sqlsentry.net/newsletter-archive. asp
- 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. 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. 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. 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: http://bit.ly/AB-cursors
- 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: http://bit.ly/AB-NOTIN
- 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 - http://bit.ly/AB-BlindIndex
- 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. 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. DEMOS: UNWANTED RECOMPILES Execution In Memory? NO Load metadata compile optimize Execute YES ReComp
- 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. 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. 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. 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. 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 http://bit.ly/15bDRRA. • Data type precedence call also have an impact: http://bit.ly/13Zio1f.
- 17. IMPLICIT CONVERSION RESOURCES • Ian Stirk’s Column Mismatch Utility at http://www.sqlservercentral.com/articles/Administration/6 5138/. • Jonathan Kehayias’ plan cache analyzer at http://sqlblog.com/blogs/jonathan_kehayias/archive/2010 /01/08/finding-implicit-column-conversions-in-the-plan-cache. aspx. • Jonathan Kehayias’ index scan study at http://www.sqlperformance.com/2013/04/t-sql-queries/ implicit-conversion-costs
- 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. 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. 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. 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. SUMMARY Let’s connect! Facebook, LinkedIn, Twitter at KEKLINE. Email at [email protected] Blogs at http://KevinEKline.com And http://ForITPros.com
- 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: http://answers.sqlperformance.com Download SQL Sentry Plan Explorer for free: http://www.sqlsentry.com/plan-explorer/ Check out our other award winning tools: http://www.sqlsentry.net/download
- 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