# Covering Indexes and Index-Only Scans in PostgreSQL

DevFeed: [Covering Indexes and Index-Only Scans in PostgreSQL](<https://devfeed.tech/articles/covering-indexes-the-cheap-10x-that-most-schemas-leave-on-the-table-39591.md>)

Original publisher: [Read original article](<https://ankit-rana.com/logs/39-covering-indexes-index-only-scans/>)

Author: hello@ankit-rana.com

Published: 2026-08-18T00:00:00Z

Content type: tutorial

Language: en

Sources: [Ankit Rana | Mechanical Sympathy](<https://devfeed.tech/sources/ankit-rana-mechanical-sympathy.md>)

Topics: [PostgreSQL](<https://devfeed.tech/topics/postgresql.md>), [Query (disambiguation)](<https://devfeed.tech/topics/query.md>), [data](<https://devfeed.tech/topics/data.md>)

Tags: [b-tree](<https://devfeed.tech/tags/b-tree.md>), [cache](<https://devfeed.tech/tags/cache.md>), [covering](<https://devfeed.tech/tags/covering.md>), [covering-index](<https://devfeed.tech/tags/covering-index.md>), [database-performance](<https://devfeed.tech/tags/database-performance.md>), [explain](<https://devfeed.tech/tags/explain.md>), [heap](<https://devfeed.tech/tags/heap.md>), [indexes](<https://devfeed.tech/tags/indexes.md>), [indexing](<https://devfeed.tech/tags/indexing.md>), [innodb](<https://devfeed.tech/tags/innodb.md>), [pages](<https://devfeed.tech/tags/pages.md>), [postgresql](<https://devfeed.tech/tags/postgresql.md>), [query](<https://devfeed.tech/tags/query.md>), [storage](<https://devfeed.tech/tags/storage.md>), [vacuum](<https://devfeed.tech/tags/vacuum.md>)

## AI overview

This tutorial explains why a normal index scan may still be slow: after finding matching entries, the database follows a pointer into the table for each row. Covering indexes store the selected columns in the index and can avoid those heap reads. In PostgreSQL, index-only scans also depend on the visibility map marking pages all-visible, while SELECT * prevents the technique from being fully effective.

## Source excerpt

A normal index scan finds matching rows and then follows a pointer into the table for every one of them, which is a random read per row. A covering index stores the columns the query selects, so the engine answers entirely from the index and skips those reads. In PostgreSQL this only works when the visibility map marks the pages all-visible, so an unvacuumed table will report Heap Fetches in EXPLAIN and give back most of the gain. SELECT star defeats the technique completely.