how to optimize a hierarchy in sql server louis davidson (drsql.org) drsql@hotmail.com

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

drsql@hotmail.com

Hierarchies

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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

ScrewPiece of Wood

Wood with Tape Screw and Tape

Tape

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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)

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…

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Working with Trees - Background

• Node Recursion

• Relational Recursion

Cycles in Hierarchies

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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

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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…

Preconceived Notions

Which method/algorithm do you expect to be fastest?

• Fixed Levels

• Adjacency List

• HierarchyId

• Path Technique

• Nested Sets

• Kimball Helper Table

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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…

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Reparenting Example

• Starting with:

• Perhaps ending with:Dragging along all of it’s child nodes along

with it

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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

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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)

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Implementing a tree – Path Method

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))

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Implementing a tree – HierarchyId

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)

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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)

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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

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

ParentId ChildId Distance ParentRootNode

ChildLeafNode

1 1 0 1 01 2 1 1 01 4 2 1 11 5 2 1 12 2 0 0 02 4 1 0 12 5 1 0 1

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.

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Demo Code

• Example code available in download

Did I change any of your minds?

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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

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Graphs are Everywhere

• Almost any many to many can be a graph

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Movie

ActorActingCast

DirectorMovieDirectory

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Graph Demo

Person

InterestPersonInterest

PersonConnection

Contact info

• Louis Davidson - louis@drsql.org

• Website – http://drsql.org <-- Get slides here

• Twitter – http://twitter.com/drsql

• SQL Blog http://sqlblog.com/blogs/louis_davidson

• Simple Talk Blog – What Counts for a DBAhttp://www.simple-talk.com/community/blogs/drsql/default.aspx