TEN QUERY TUNING TECHNIQUES

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Esmond McCoy
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1 TEN QUERY TUNING TECHNIQUES Every SQL Programmer Should Know Kevin Kline Director of Engineering Services at SentryOne Microsoft MVP since 2003 Facebook, LinkedIn, Twitter at KEKLINE

2 Tuning blog: for free copies of our e-books:

3 Agenda Introductions Test & tuning environment 1. Clearing caches Looking for red flags 2. Reading execution plans Query tuning techniques: 10 more specific examples of widespread approaches that lead to poor performance Summary

4 A Reliable Test Harness Code to clear the caches*: CHECKPOINT DBCC [FreeProcCache FreeSystemCache FlushProcInDB(<dbid>) ] DBCC DropCleanBuffers Code to set measurements: SET STATISTICS [TIME IO] SET SHOWPLAN [TEXT XML] or Graphic Execution Plans

5 Mimicking production Your dev machine is usually nothing like production Build a stats-only database when you can (a.k.a. a database clone) Build representative data when you can Simulate equivalent hardware: DBCC OPTIMIZER_WHATIF

6 Red Flags in your SQL Code Red Flags Plan Operators: Lookups, Scans, Spools, Parallelism Operations Other Red Flags: Dissimilar estimated versus actual row counts High physical reads Missing statistics alarms Large sort operations Implicit data type conversions

7 1. Good Cursors? I don t always use cursors but when I do, I avoid the default options Slow and heavy-handed: Global, updateable, dynamic, scrollable I use LOCAL FAST_FORWARD May want to test STATIC vs. DYNAMIC, when tempdb is a bottleneck Blog post: DEMO

8 2. 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:

9 3. Optimizing For SELECT versus DML (Insert, update, & Delete) 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 -

10 4. Sargs: Reads & Index Structure 8K pages Leaf pages ARE the data. Non-leaf pages are pointers. Root Page Level 2 Intermediate Pages Level 1 Leaf Pages Level 0

11 Writes & Index Structure Each change to the leaf pages requires all index structures be updated. Intermediate Pages Root Page Level 2 Level 1 Leaf Pages Level 0 Actual placement DML Page Split

12 5. Unwanted Recompiles Execution In Memory? YES NO ReComp Load metadata compile optimize Execute

13 CODE RecompileS WHEN 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

14 EXAMPLE: 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>

15 6. The "kitchen sink" procedure Usually see it as a one-query-for-all-queries procedure, or even one-proc-for-for-all-transactions procedure: Where name starts with S, or placed an order this year, or lives in Texas Insert AND Update AND Delete AND Select Conflicting optional parameters make optimization impossible OPTION (RECOMPILE) Dynamic SQL + Optimize for ad hoc workloads Specialized procedures Better approach? Specialize and optimize each piece of code to do ONE THING really effectively

16 7. sp_executesql vs. EXEC( ) I don t always use dynamic SQL but when I do, I always use sp_executesql Less fuss with concatenation and implicit/explicit conversions Better protection against SQL injection (but not for all things) At worst case, behavior is the same Can promote better plan re-use Encourages strongly typed parameters instead of building up a massive string. Helps prevent SQL injection.

17 8. 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:

18 Implicit conversion resources Ian Stirk s Column Mismatch Utility at /. Jonathan Kehayias plan cache analyzer at 0/01/08/finding-implicit-column-conversions-in-the-plancache.aspx. Jonathan Kehayias index scan study at

19 9. 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 10. Coding standards and Dissimilarity Might sound frivolous, but standards and naming schemes are important! 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 Object resolution works a little bit harder without it More importantly, it can get the wrong answer And will often yield multiple copies of the same plan Do not use the sp_ prefix on stored procedures This has observable overhead, no matter how specific you are

21 A Final Test dbo.sptest (Kev) Exec sptest select * from test dbo.test Kev.test dbo stuff Kev stuff

22 SUMMARY Let s connect! Facebook, LinkedIn, Twitter at KEKLINE. at KEKline@sqlsentry.com Blogs at

23 Thank you! Join me tomorrow for two more sessions of the SSWUG vconference! Slides and scripts at and on here.

SELECT TOP (?) * FROM [50 Things All SQL Server Developers Need To Know] Aaron Bertrand, Senior Consultant

SELECT TOP (?) * FROM [50 Things All SQL Server Developers Need To Know] Aaron Bertrand, Senior Consultant SQL Sentry, LLC Thank You Presenting Sponsors Gain insights through familiar tools while balancing