Choosing the Right Hierarchical Data Model for Performance

When it comes to managing hierarchical data in databases, developers often face a crucial decision: which data model to employ for optimal performance. Two popular options in this arena are the Adjacency List Model and the Nested Set Model. In this article, we'll delve into these models, exploring their characteristics and performance implications to help you make an informed choice.

Understanding the Adjacency List Model

The Adjacency List Model is perhaps the simplest and most intuitive approach to representing hierarchical data. In this model, each record contains a reference to its parent record, usually through a "parent_id" column. Let's illustrate this with a familiar example: a category hierarchy.

Consider an e-commerce platform where products are organized into categories. Using the Adjacency List Model, the category table might look something like this:

category_id	category_name	parent_id
1	Electronics	NULL
2	Clothing	NULL
3	Phones	1
4	Laptops	1
5	T-shirts	2
6	Jeans	2
7	iPhone	3
8	Samsung	3
9	Dell	4
10	HP	4

In this example, categories like "Phones" and "Laptops" have a parent category "Electronics", and categories like "iPhone" and "Samsung" have a parent category "Phones".

The simplicity of the Adjacency List Model makes it easy to understand and implement. However, its performance may degrade with deeply nested structures or when performing operations like subtree traversal.

Exploring the Nested Set Model

In contrast to the Adjacency List Model, the Nested Set Model represents the hierarchy as a nested set of intervals within a single table. Each node in the tree is assigned a left and right value such that the left value of a node is less than the left value of its children, and the right value is greater.

Let's continue our example of the category hierarchy using the Nested Set Model:

category_id	category_name	left_value	right_value
1	Shop	1	22
2	Clothing	2	7
3	T-shirts	3	4
4	Jeans	5	6
5	Electronics	8	21
6	Phones	9	14
7	iPhone	10	11
8	Samsung	12	13
9	Laptops	15	20
10	Dell	16	17
11	HP	18	19

In this model, each category is represented by a pair of left and right values, defining its position within the hierarchy. For example, the "Electronics" category spans from left_value 8 to right_value 21.  The graph representation below helps illustrate the hierarchical structure of the data

While the Nested Set Model may seem more complex to grasp initially, it offers significant performance benefits, especially for operations like subtree traversal, finding ancestors or descendants, and determining the depth of nodes.

Comparison Table:

Aspect	Adjacency List Model	Nested Set Model
Implementation	Simple	More Complex
Writing/Updating	Fast	Slower
Reading	Slower	Fast
Traversal	Slower	Fast
Finding Descendants	Slower	Fast
Finding Ancestors	Slower	Fast
Maintenance Effort	Low	Moderate to High

This table provides a concise overview of the strengths and weaknesses of each model in various aspects. It's important to note that the suitability of each model depends on the specific requirements and priorities of your application.

While the Adjacency List Model may inherently be slower for certain types of hierarchical data operations like reading, finding descendants, or finding ancestors due to its structure, modern relational database systems like MySQL/MariaDB offer features like recursive queries or recursive common table expressions (CTEs) that can significantly improve performance for these operations (with some downsides). But we will mention about this on the next article.