---
title: "All Your GUCs in a Row: enable_indexonlyscan"
url: https://daily.dev/posts/all-your-gucs-in-a-row-enable-indexonlyscan-s3vvm0cff
source_url: https://postgr.es/p/9nM
type: article
source: "Planet PostgreSQL"
published: 2026-06-30T01:01:24.908Z
updated: 2026-06-30T01:01:49.451Z
tags: ["postgresql"]
reading_time: 5
upvotes: 0
comments: 0
language: en
---

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# All Your GUCs in a Row: enable_indexonlyscan

**[Planet PostgreSQL](https://daily.dev/sources/planet-postgresql)** · 5 min read · 0 upvotes · 0 comments

## Summary

PostgreSQL's index-only scan can skip heap reads entirely when every needed column is in the index, potentially delivering 2-20x speedups. However, the critical catch is that MVCC visibility information lives in the heap, not the index. The visibility map (a per-table bitmap maintained by VACUUM) is what makes index-only scans actually work: if a heap page's all-visible bit is set, no heap fetch is needed. On actively-changing tables where autovacuum can't keep up, an index-only scan plan can silently degrade into heavy heap I/O. The diagnostic tool is EXPLAIN (ANALYZE)'s 'Heap Fetches:' counter — a high count means a stale visibility map, and the fix is tuning autovacuum (lower autovacuum_vacuum_scale_factor), not disabling the GUC. The enable_indexonlyscan GUC is a diagnostic instrument: turn it off temporarily to compare plans, not a permanent tuning knob.

## Full article

daily.dev links to this article rather than hosting it. Read it at the original source: <https://postgr.es/p/9nM>

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---

Tags: [#postgresql](https://daily.dev/tags/postgresql)

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