Lesson 3 of 12

B-Tree Indexes: Structure and When They Win

Applies to PostgreSQL 13–17 Last reviewed May 2026 Grounded in source

The one thing to understand first

A B-tree is the default and most versatile PostgreSQL index, implemented in src/backend/access/nbtree/. It supports equality and range operators (=, <, <=, >, >=, BETWEEN, IN), ORDER BY acceleration, and uniqueness enforcement — all from one balanced, ordered structure.

A B-tree wins because it keeps your keys sorted — and almost everything useful (selective equality, ranges, ORDER BY, uniqueness) is just a consequence of sorted order. Understand that, plus the leftmost-prefix rule for composite indexes, and most “why isn’t my index used” questions answer themselves.

Structure: a balanced ordered tree

PostgreSQL implements a Lehman-Yao B-tree variant designed for high concurrency. The tree has:

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