1. Cyndi Johnson
Senior Software Engineer at AdvancedMD
Killing Cursors

2. Killing Cursors Techniques to Eradicate/Improve Cursors & Other Loops Within Your Database Code

4. Overview Why this session? Why are cursors/loops bad?
Alternative approaches
Business Use Case & Refactored Solutions
Tips & Considerations

5. Why this session?
<Talking> blah blah blah </Talking>

6. In This Demo, I’m using: SQL Server 2016 SSMS v17.4
SQL Sentry Plan Explorer V.3 (FREE!)

7. Why are cursors/loops bad?
Cursors/loops are an imperative approach- you’re telling the engine (compiler,etc) how to do its job
T-SQL is DECLARATIVE- tell the engine what you need and it will decide how best to get there.
Cursors/loops usually do not utilize set-based approaches in a way that is efficient and will scale
There is always another way
Discuss the flaw of debating CURSORS VS. LOOPS, point to Aaron Bertrand’s blog post on the subject

8. Alternative Approaches
Easy problems solved with cursors/loops (not covered during this session)
Calling existing stored procedures within a loop
use bulk INSERTS while utilizing OUTPUT clause to capture identity fields and act using those captured fields
string concatenations
use FOR XML instead
Harder problems often solved with cursors/loops
Hierarchical data relationships
application of payments in accounting
overlapping worklists

9. Business Use Case- Overlapping Priority Driven Worklists: One charge may fit the definition for several of the worklist’s filter definitions, but the one with the highest priority wins W1 W3 W2

10. There is much value to understanding the data

11. While Loop Basic Syntax

12. DEMOS Original Version
Show CURSOR_NonDynamic & CURSOR_Dynamic stored procedures

13. DEATH BY A THOUSAND CUTS
Or not seeing the forest through the trees

14. Refactored V.1 Approach-Basic CTE
Show Worklist_Queries stored procedure
ASIDE- What do CTEs give you that derived tables/subqueries do not?

15. Refactored V.1
DEMOS
Show CTE_Normal_Dynamic stored procedure

16. Refactored V.2 Approach-Recursive CTE
Default MAXRECURSION is 100
Show PERFECT_WORLD.sql

17. SO MANY RULES! Recursive CTE Rules
From
At least one ANCHOR member and one RECURSIVE member
UNION ALL must be used between the last ANCHOR and the first RECURSIVE member
Date types MUST match exactly between the ANCHOR and RECURSIVE member
RECURSIVE member FROM clause may only refer to CTE expression name ONCE
SO MANY RULES!

18. Refactored V.2
DEMOS
Show CTE_Recursive stored procedure

19. WHILE LOOP using CTE OVER() PARTITION BY() Dynamic SQL
And the WINNER is…
WHILE LOOP using CTE OVER() PARTITION BY() Dynamic SQL

20. Why did Recursive CTE not perform?
No unique parent/child key
Complex predicates (conditional filters)
Recursive CTEs CAN perform well in the right circumstances
Discuss anchor member VS several recursive members of the CTE & why the steps weren’t ideal (NULL case had to be added)

21. What is a good use case for Recursive CTE?
Unique parent/child key
Simple predicates
Smaller data set to operate on whether in a table or a temp table, fields used to step through the data are properly indexed

22. Tips & Considerations
Know your data- understanding the data will help you know how to improve upon the processes
If you don’t know the data- TEST TEST TEST. You should do this regardless
Execution plans are your friend! Get in the habit of looking at them often, even if you don’t understand them very well. That will come with time.
If you have to leave a cursor/loop intact, is there any way you can reduce the times it goes through the loop?

23. Additional Resources
Fritchey G. (2012) Cursor Cost Analysis. In: SQL Server 2012 Query Performance Tuning. Apress, Berkeley, CA

24. Thank You! linkedin.com/in/cjdatawhisperer medium.com/@SQLanodyne

25. Files Are Huge! https://1drv.ms/u/s!Aimffvkpm1Eauw2N5lb0L7hUWvH-

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