Recursive Query Fundamentals

Recursive queriees enable hierarchical data processing by referencing the query itself during execution. Starting from version 5.7, MySQL supports recursive queries through Common Table Expressions (CTE) with the WITH RECURSIVE clause. This capability is particularly useful for managing tree-structured data such as organizational hierarchies or category trees.

Consider a categories table with the following schema:

• id - Unique identifier for the category
• parent_id - Reference to the parent category
• title - Name of the category

A basic recursive queery to retrieve the full tree structure would look like this:

WITH RECURSIVE child_categories AS (
  SELECT id, parent_id, title
  FROM categories
  WHERE parent_id IS NULL
  UNION ALL
  SELECT c.id, c.parent_id, c.title
  FROM categories c
  INNER JOIN child_categories cc ON cc.id = c.parent_id
)
SELECT * FROM child_categories;

Tree Pagination Strategy

Paginating hierarchical data presents unique challenges. A standard LIMIT/OFFSET approach may break the logical hierarchy by splitting parent-child relationships across pages. A better solution involves using a traversal algorithm that maintains the structural integrity during pagination.

The pre-order traversal technique assigns a sequential number to each node based on its position in a depth-first traversal. This sequence number enables structured pagination while preserving the tree relationships.

Paginated Recursive Query Implementation

The following query demonstrates how to implement hierarchical pagination using recursive CTEs and sequence numbering:

WITH RECURSIVE hierarchy_tree AS (
  SELECT 
    id, 
    parent_id, 
    title, 
    1 AS level, 
    CAST(id AS CHAR(100)) AS hierarchy_path
  FROM categories
  WHERE parent_id IS NULL
  UNION ALL
  SELECT 
    c.id, 
    c.parent_id, 
    c.title, 
    ht.level + 1, 
    CONCAT(ht.hierarchy_path, ',', c.id)
  FROM categories c
  INNER JOIN hierarchy_tree ht ON ht.id = c.parent_id
),
ordered_tree AS (
  SELECT 
    id, 
    parent_id, 
    title, 
    level, 
    hierarchy_path,
    ROW_NUMBER() OVER (ORDER BY hierarchy_path) AS node_order
  FROM hierarchy_tree
)
SELECT * FROM ordered_tree
WHERE node_order BETWEEN 11 AND 20;

This query performs the following operations:

1. Builds the hierarchy using recursive CTE
2. Tracks node depth and constructs a traversal path
3. Assigns sequence numbers based on traversal order
4. Selects a specific range for pagination

Performance Considerations

• Caching - For static or infrequently updated trees, cache the sequence numbers to avoid repeated recursive computation
• Materialized Views - Consider storing traversal paths and sequence numbers in dedicated columns for large datasets
• Update Management - Implement triggers or application logic to maintain sequence values when tree structure changes