Cookbook recipe

How a b-tree index speeds up a lookup

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

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A production experience capsule based on real incident patterns.

Why It Matters

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Estimated investigation4 min

Scenario

You hear "add an index" constantly but want to know what actually makes it fast. The b-tree turns a full scan into a few page reads. Diagnose it Compare a scan with and without an index:…

Investigation Path

You hear “add an index” constantly but want to know what actually makes it fast. The b-tree turns a full scan into a few page reads.

Diagnose it

Compare a scan with and without an index:

EXPLAIN SELECT * FROM users WHERE email = '[email protected]';  -- Seq Scan
CREATE INDEX ON users (email);
EXPLAIN SELECT * FROM users WHERE email = '[email protected]';  -- Index Scan

Why it happens

A b-tree keeps keys sorted in a shallow, balanced tree. Finding a value walks from the root down a handful of levels — O(log n) page reads — instead of reading every row.

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Related & next steps

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