SQL SERVER QUERY TUNING BEST PRACTICES, PART 5 OF 6

Aaron BertrandSQL Sentry, Senior Consultant@AaronBertrand

Kevin KlineSQL Sentry, Dir of Engineering Services@KEKline

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AGENDA• Introductions• Patterns & Anti-Patterns

o The “Kitchen Sink” stored procedureo SP_ExecuteSQL vs EXEC(…)o Execution Plan Self-Destructo To MERGE or not to MERGE

• Prizes!• Follow Up

SQL PATTERNS AND ANTI-PATTERNS13. Specifying the schema14. SP_xyz Prefix15. Unwanted recompiles16. The “Kitchen Sink” stored procedure17. SP_ExecuteSQL vs EXEC(…)18. Execution Plan Self-Destruct19. To MERGE or not to MERGE

1. Bad, Naughty Default Cursors2. Correlated Subqueries3. WHERE IN versus WHERE EXISTS4. UNION versus UNION ALL5. WHERE {NOT IN | EXISTS} versus

LEFT JOIN6. Queries optimized for SELECT but not DML

statements7. Compound index columns8. Covering indexes9. The Transitive Property10. Queries with IN (…)or OR 11. Queries with wildcard searches12. Using functions in WHERE or JOIN clauses

THE "KITCHEN SINK" PROCEDURE• Many optional parameters to satisfy a variety of search

conditions:o Dynamic SQL is often the best route hereo Especially if Optimize For Ad Hoc Workloads is enabledo Could also use RECOMPILE, but that means you pay compile cost every time

• DEMO

SP_EXECUTESQL VS. EXEC(…)• Can promote better plan re-use• Encourages strongly typed parameters instead of

building up a massive string• DEMO

EXECUTION PLAN SELF-DESTRUCT• Execution plans are intended to be reused as much as

makes good sense.• There are a variety of hidden or inconspicuous reasons

that execution plans will self-destruct and never be reused:o Contradictory language settingso Contradictory collation settings, especially with Unicodeo Contradictory SET optionso Users with different default schemas not using schema prefix

CONTRADICTIONS• When SQL Server is faced with contradictory settings, it

will usually opt to generate a new execution plan:o If language settings are different on client and server, even at the Windows-level

settings.o If collation settings differ between servers, server and client, or between a

Transact-SQL batch and the objects they are operating upon. Especially problematic with Unicode.

SET OPTION HIERARCHY1. SET statement in code2. OLEDB/ODBC connection

string3. ODBC

o Control Panelo SQLConfigDatasource

4. OLEDB/ODBC option auto set 5. Database Level Settings (Alter

Database)6. Server Wide Settings

(sp_configure)

SET Option Requirements:

ON OFF Set-able By:

Arithabort s S,D,A

Concat_null_yields_null s S,D,A

Quoted_Identifier** s S,D,A

Ansi_nulls** sp S,D,A

Ansi_Padding s S,D,A

Ansi_Warnings sp S,D,A

Numeric_roundabort s S,D,A

Forceplan SET only

Ansi_Null_Dflt_On S,D,A

Ansi_Null_Dflt_Off S,D,A

s = required for index views or computed columnsp = required for distributed / linked queriesS, D, A = sp_configure, sp_dboption, Alter Database

TO MERGE OR NOT TO MERGE?• Unnecessarily complex syntax• Can complicate multi-operation trigger logic• By default, does *NOT* promise concurrency protection

or prevent race conditionso In the end it is not really any more efficient than separate statements

• Many unresolved bugs• More details at http://bit.ly/AB-vs-MERGE 

SUMMARY• The “Kitchen Sink” stored procedure is bad. Specialize

your stored procedures• SP_ExecuteSQL vs EXEC(…) have different strengths

and weaknesses. Know when to use each.• Execution Plans can Self-Destruct due to hidden or

inconspicuous settings.• To MERGE or not to MERGE? It’s not as clear cut as

you might think.

FOLLOW UP1. Engage with our community: SQL Sentry on

Facebook, SQLSentry.Net, SQLPerformance.com2. Share your tough query problems with us:

http://answers.sqlperformance.com3. Download SQL Sentry Plan Explorer for free:

http://www.sqlsentry.net/plan-explorer/4. Check out our other award winning tools: http://

www.sqlsentry.net/download