HOW TO OPTIMIZE A HIERARCHY IN SQL SERVER Louis Davidson (drsql.org)

Who am I? Been in IT for over 18 years Microsoft MVP For 9 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

Hierarchies 3

Hierarchy Types 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

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) 5

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… 6

Working with Trees - Background Node Recursion Relational Recursion 7

Cycles in Hierarchies 8 Parent Child “I’m my own grandpa” syndrome Must be understood or can cause infinite loop in processing Generally disallowed in trees Generally handled in graphs Grandparent

Tree Processing Algorithms There are several methods for processing trees in SQL We will cover 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

Preconceived Notions Which method/algorithm do you expect to be fastest? Fixed Levels Adjacency List HierarchyId Path Technique Nested Sets Kimball Helper Table 10

Coding for trees Manipulation: Creating a new node Moving/Reparenting a node Deleting a node (with/without children) Usage Getting the children of a node Getting the parent of a node Aggregating along the tree Note: No tree algorithms allow for “simple” SQL solutions to all of these problems We will have demos of all of these operations… 11

Reparenting Example Starting with: Perhaps ending with: 12 Dragging along all of it’s child nodes along with it

Implementing a tree – Fixed Levels CREATE TABLE CompanyHierarchy ( Company varchar(100) NULL, Headquarters varchar(100) NULL, Branch varchar(100) 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 13

Implementing a tree – Adjacency List Every row includes the key value of the parent in the row Parent-less rows have no parent value Code to get information out 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 ) 14

Implementing a tree – Path Method 15 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) )

Implementing a tree – HierarchyId 16 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 )

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 ) 17

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 18

Implementing a tree – Kimball Helper For the rows in yellow, expands to the table shown: 19 ParentIdChildIdDistanceParentRootNodeChildLeafNode

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, Delete, and Reparenting a Node Queries to access and aggregate the data in the hierarchy We will do this for two sets of data, the data in the presentation, and then a randomly generated set. 20

Demo Code Example code available in download 21

Did I change any of your minds? 22

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 23

Graphs are Everywhere Almost any many to many can be a graph 24 Movie ActorActingCast DirectorMovieDirectory

Graph Demo 25 Person InterestPersonInterest PersonConnection

Contact info Louis Davidson - Website – <-- Get slides herehttp://drsql.org Twitter – SQL Blog Simple Talk Blog – What Counts for a DBA