JOIN Algorithms

February 24, 2026

Understanding Nested Loop, Hash Join, and Merge Join in database engines.

When you execute a SQL JOIN, the database engine has several strategies at its disposal.

The choice of algorithm can make the difference between a query taking milliseconds or minutes.

  1. Nested Loop Join
  2. Hash Join
  3. Merge Join

Each has its strengths and weaknesses depending on the size of the tables and available indexes.