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

kkline@sentryone.com

Tuning blog: http://www.sqlperformance.com/

E-mail ebooks@sqlsentry.com for free copies of our e-books:

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

3

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

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

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

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: http://bit.ly/AB-cursors

• DEMO

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: http://bit.ly/AB-NOTIN

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 - http://bit.ly/AB-BlindIndex

4. Sargs: 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

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

5. Unwanted Recompiles

Execution

Load metadata NO In Memory?

compile

optimize

Execute

YES

ReComp

Execute

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

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>

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

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.

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 http://bit.ly/15bDRRA.

• Data type precedence call also have an impact:

http://bit.ly/13Zio1f.

Implicit conversion resources • Ian Stirk’s Column Mismatch Utility at

http://www.sqlservercentral.com/articles/Administration/65138/.

• 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

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

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

A Final Test

dbo stuff

dbo.test

Kev stuff

Kev.test

select * from test

dbo.sptest

(Kev) Exec sptest

SUMMARY

Let’s connect!

Facebook, LinkedIn, Twitter

at KEKLINE.

Email at

KEKline@sqlsentry.com

Blogs at

http://KevinEKline.com

Thank you! • Join me tomorrow for two more sessions of the SSWUG

vConference!

• Slides and scripts at http://slideshare.net/kkline84 and on

here.