When it comes to database performance optimization, proper indexing is crucial. While single-column indexes are straightforward, compound indexes (als

Understanding PostgreSQL Compound Indexes: A Deep Dive

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2025-01-20 11:30:08

When it comes to database performance optimization, proper indexing is crucial. While single-column indexes are straightforward, compound indexes (also known as composite or multi-column indexes) require a deeper understanding. In this post, we’ll explore how compound indexes work in PostgreSQL, when to use them, and how to optimize your queries.

A compound index is an index on multiple columns of a table. Think of it like a phone book organized by last name and then first name. The order of columns in the index definition is crucial for how PostgreSQL can use it.

The key principle is that PostgreSQL can only use index columns from left to right. Let’s examine different query patterns and their effectiveness:

This can only use the userid part of the index. The age condition must be checked separately because we skipped the email column.

One powerful feature of compound indexes is their ability to support index-only scans. If all columns needed by a query are in the index, PostgreSQL can retrieve data directly from the index without touching the table.