postgres-syntax-lateral-joins

What problem does it solve?

This Skill addresses the challenges of using LATERAL joins in PostgreSQL for dynamic subqueries, correlated data, and complex join scenarios, providing guidance to prevent common mistakes and improve query performance.

Core Features & Use Cases

• Dynamic Subquery Execution: Facilitates per-row dynamic subqueries and avoids losing outer rows when LATERAL subqueries return empty.
• Correlated Subquery Handling: Offers strategies for handling correlated subqueries, including equivalence with non-lateral joins and best practices for top-N-per-group results.
• Use Case: For example, it helps in creating a query that computes the 3 most recent posts per user, even if some users have no posts, by using LATERAL join effectively.

Quick Start

Run the query: SELECT * FROM users u LEFT JOIN LATERAL (SELECT id, title, created_at FROM posts WHERE posts.user_id = u.id ORDER BY created_at DESC LIMIT 3) AS p ON true;