SQL SERVER QUERY TUNING BEST PRACTICES, PART 6 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 Efficient ad hoc sets (Aaron)o Best practices for comma-delimited parameters (Aaron)o Implicit Conversions (Kevin)o Best practices in temporary structures (Kevin)

• 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 MERGE20. Efficient ad hoc sets 21. Implicit conversions 22. Best practices for comma-delimited

parameters 23. Best practices in temporary structures

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

GENERATE AD-HOC SETS EFFICIENTLY

• Example: a set of days in a range• Recursive CTEs can do this, but…• Create a permanent number/calendar table instead

• DEMO

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.

• DEMO

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.sqlskills.com/blogs/jonathan/implicit-conversions-that-cause-index-scans/

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

• DEMO

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

SUMMARY• Comma-delimited parameters may be better with TVPs

than splitting strings.• Implicit conversions can cause extra CPU work and/or

index and table scans.• Efficient ad hoc sets may not be best in recursive CTEs.

Test your alternatives.• Temporary tables and temporary variable each have

their uses. Know when to use each.

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