1. drsql.org How to Implement a Hierarchy in SQL Server Louis Davidson (drsql.org) drsql@hotmail.com

2. drsql.org Who am I? Been in IT for over 19 years Microsoft MVP For 10 Years Corporate Data Architect Written five books on database design – Ok, so they were all versions of the same book. They at least had slightly different titles each time

3. drsql.org 3 Hierarchies 3

4. drsql.org 4 Hierarchies Trees - Single Parent Hierarchies Graphs – Multi Parent Hierarchies – Note: Graphs can be complex to deal with as a whole, but often you can deal with them as a set of trees 4 Screw Piece of Wood Wood with TapeScrew and Tape Tape

5. drsql.org 5 Cycles in Hierarchies 5 Parent Child “I’m my own grandpa” syndrome Must be understood or can cause infinite loop in processing Generally disallowed in trees May be supported in graphs, particularly for establishing relationships Grandparent

6. drsql.org 6 Hierarchy Uses Trees – Species – Jurisdictions – “Simple” Organizational Charts (Or at least the base manager-employee part of the organization) – Directory folders Graph – Bill of materials – Complex Organization Chart (all those dotted lines!) – Genealogies Biological (Typically with limit cardinality of parents to 2 ) Family Tree – (Sky is the limit) – Social Networking Relationships Example: (Bob is connected to Sue, Sue is connected to Fred, Fred is connected to Bob) 6

7. drsql.org 7 Implementation of a Hierarchy “There is more than one way to shave a dog” – None of which are pleasant for the dog or the shaver – And the doctor who orders it only asks for a bald dog Hierarchies are not at all natural to manipulate/query using relational code – And the natural, recursive processing of a node at a time is horribly difficult and slow in relational code – So, multiple methods of processing them have arisen through the years The topic (much like the topic of how cruel it is to shave a dog), inspires religious-like arguments I find all of the implementation possibilities fascinating, so I set out to do an overview of them all… 7

8. drsql.org 8 Working with Trees - Background Node recursion Relational Recursion 8

9. drsql.org 9 Tree Processing Algorithms There are several methods for processing trees in SQL We will look at – Fixed Levels – Adjacency List – HierarchyId – Path Technique – Nested Sets – Kimball Helper Table Without giving away too much, pretty much all of the methods have some use… 9

10. drsql.org 10 Coding for trees Manipulation: – Creating a new node – Moving/Reparenting a node – Deleting a node (without children) – Note: No tree algorithms allow for “simple” SQL solutions to all of these problems Usage – Getting the children of a node – Getting the parent of a node – Aggregating along the tree We will have demos of all of these operations…available at least 10

11. drsql.org 11 Reparenting Example Starting with: Perhaps ending with: 11 Dragging along all of it’s child nodes along with it

12. drsql.org 12 Implementing a tree – Fixed Levels CREATE TABLE CompanyHierarchy ( Company varchar(100) NOT NULL, Headquarters varchar(100) NOT NULL, Branch varchar(100) NOT NULL, PRIMARY KEY (Company, Headquarters, Branch) ) Very limited, but very fast and easy to work with I will not demo this structure today because it’s use is both extremely obvious and limited 12

13. drsql.org 13 Implementing a tree – Adjacency List Every row includes the key value of the parent in the row Parent-less rows have NULL parent value Code is the most complex to write (though not as inefficient as it might seem) CREATE TABLE CompanyHierarchy ( Organization varchar(100) NOT NULL PRIMARY KEY, ParentOrganization varchar(100) NULL REFERENCES CompanyHierarchy (Organization), Name varchar(100) NOT NULL ) 13

14. drsql.org 14 Adjacency List – Adding a Node 14 New Node

15. drsql.org 15

16. drsql.org 16 Simply set the parent and done!

17. drsql.org 17 Implementing a tree – Path Method 17 Every row includes a representation of the path to their parent Processing makes use of like and string processing ( I have seen a case that used fixed length binary values) Limitation on path size for string manipulation/indexing CREATE TABLE CompanyHierarchy ( OrganizationId int NOT NULL PRIMARY KEY, Name varchar(100) NOT NULL, Path varchar(900) ) 900 Bytes allows for indexed manipulations

18. drsql.org Path Method Adding a Node 18 New Node

19. drsql.org 19 New Id = 9

20. drsql.org 20 Plus the New Id Path from the parent

21. drsql.org 21 Implementing a tree – Path Method 21 Every row includes a representation of the path to their parent Processing makes use of like and string processing ( I have seen a case that used fixed length binary values) Limitation on path size for string manipulation/indexing CREATE TABLE CompanyHierarchy ( OrganizationId int NOT NULL PRIMARY KEY, Name varchar(100) NOT NULL, Path varchar(900) )

22. drsql.org 22 Implementing a tree – HierarchyId 22 Somewhat unnatural method to the typical SQL Programmer Similar to the Path Method, and has some of the same limitations when moving around nodes Node path does not use data natural to the table, but rather positional locationing CREATE TABLE CompanyHierarchy ( OrganizationId int NOT NULL PRIMARY KEY, Name varchar(100) NOT NULL, OrgNode hierarchyId not null )

23. drsql.org 23 Implementing a tree – Nested Sets Query processing is done using range queries Structure is quite slow to maintain due to fragile structure Can produce excellent performance for queries CREATE TABLE CompanyHierarchy ( Organization varchar(100) NOT NULL PRIMARY KEY, Name varchar(100) NOT NULL, Left int NOT NULL, Right int NOT NULL ) 23

24. drsql.org New Node Nested Sets – Adding a Node

25. drsql.org Updating Right Values

26. drsql.org And the One Left value right of the new node

27. drsql.org Renumber, leaving gap for child

28. drsql.org The New Node

29. drsql.org Set the New Node’s Left/Right

30. drsql.org 30 Implementing a tree – Nested Sets Query processing is done using range queries Structure is quite slow to maintain due to fragile structure Can produce excellent performance for queries CREATE TABLE CompanyHierarchy ( Organization varchar(100) NOT NULL PRIMARY KEY, Name varchar(100) NOT NULL, Left int NOT NULL, Right int NOT NULL ) 30

31. drsql.org 31 Implementing a tree – Kimball Helper Developed initially for data warehousing since data is modified all at once with a fixed cost Basically explodes the hierarchy into a table that turns all hierarchy manipulations into a relational query Maintenance can be slightly costly, but using the data is extremely fast 31

32. drsql.org 32 Implementing a tree – Kimball Helper For the rows in yellow, expands to the table shown: 32 ParentIdChildIdDistanceParentRootNodeChildLeafNode 11010 12110 14211 15211 22000 24101 25101

33. drsql.org 33 Performance Examples and Limitations The following tests were run multiple times, and the results were taken from one such run. Clearly the results are not scientific, and done with random data. However, they very much match my expectations from my research. Load times were captured loading one row at a time. Test machine (this laptop I am using tonight) was a: – Lenovo Yoga Pro 2, Haswell ULT i7 (4 th Gen Intel Mobile Processor), 2.4Ghz Dual Core (Hyperthreaded), 8GB RAM, 256 GB SSD Note: All load times include time to load 5 transactions per node 33

34. drsql.org 34 Performance Example Explanation For each performance test (which I will show the code later), I ran three query sets on each data set: 1.Load the tree (until my computer couldn’t do it in a reasonable number of hours) 2.Fetch all children from the root node 3.Aggregate data for all children at all levels 34

35. drsql.org 35 Performance Comparisons 35

36. drsql.org 36 Performance Comparisons 36

37. drsql.org 37 Performance Comparisons 37

38. drsql.org 38 Performance Comparisons 38

39. drsql.org 39 Performance Comparisons 39

40. drsql.org 40 Performance Comparisons 40

41. drsql.org 41 Performance Comparisons 41

42. drsql.org 42 Performance Comparisons 42

43. drsql.org 43 Performance Comparisons 43

44. drsql.org 44 Performance Comparisons 44

45. drsql.org 45 Performance Comparisons 45

46. drsql.org 46 Method Comparison 46

47. drsql.org 47 Demo Code Example code for all examples available for download. Will demo hierarchies and graphs. 47

48. drsql.org 48 Method Applicability Method -> Applicability Adjacency List HierarchyIdPathMethodNestedSetKimball Helper General Purpose Hierarchies *** * VERY Large Hierarchy Queries ******* Offline Reporting ****** (Cost of maintaining limits use) *** OLTP Use***** ** (Perhaps slower to load nodes) Highly Concurrent Modification ******* Highly Concurrent Queries ****** Unlimited Hierarchy Size ** * (Width unlimited, Effective depth limited by 900 byte index limit) *** 48

49. drsql.org 49 Future Improvements Use SQL Server 2014 In-Memory Database to help with locking and brute force operations Adjust Nested Sets to use fractional numbers to reduce load time costs Load an order of magnitude more data Try these examples on a “real” computer! 49

50. drsql.org 50 Graphs Generally implemented in same manner as adjacency list – Can be processed in the same manner as an adjacency list – Primary difference is child can have > 1 parent node – Cycles are generally acceptable Graph structure will always be external to data structure Graphs are even more natural data structures than trees 50

51. drsql.org 51 Graphs are Everywhere Almost any many to many can be a graph 51 Movie ActorActingCast DirectorMovieDirector

52. drsql.org 52 Demo Setup For each style of hierarchy, we will see how to: – Implement a physical model that models the corporate hierarchy of the previous graphics – Create Stored Procedures for Insert, Reparenting, Deleting Data – Queries to access and aggregate the data in the hierarchy 52

53. drsql.org 53 Demo Code Example code for all examples available for download. Will demo hierarchies and graphs. 53

54. drsql.org 54 Contact info Louis Davidson - louis@drsql.orglouis@drsql.org Website – http://drsql.org <-- Get slides herehttp://drsql.org Twitter – http://twitter.com/drsqlhttp://twitter.com/drsql SQL Blog http://sqlblog.com/blogs/louis_davidsonhttp://sqlblog.com/blogs/louis_davidson Simple Talk Blog – What Counts for a DBA http://www.simple-talk.com/community/blogs/drsql/default.aspx http://www.simple-talk.com/community/blogs/drsql/default.aspx

55. drsql.org Thank you That’s all folks! 55