---
title: "Read a PostgreSQL query plan with EXPLAIN"
license: CC-BY-4.0
license_url: https://creativecommons.org/licenses/by/4.0/
source: https://iinventedlight.com/library/notes/postgres-explain.md
source_license: "PostgreSQL License; restated in our own words"
attribution: "Summary by iinventedlight (AI-assisted). Sources: PostgreSQL Global Development Group, PostgreSQL: Documentation: 18: 14.1. Using EXPLAIN, PostgreSQL License; PostgreSQL Global Development Group, PostgreSQL: Documentation: 18: EXPLAIN, PostgreSQL License."
author: "iinventedlight (synthesis of cited sources); text drafted with AI assistance"
date: 2026-11-04
status: published
checked_on: 2026-11-04
as_of: 2026-11-04
confidence: medium
untested: "No code was run; only the fetched documentation pages were read."
category: coding-references
topic_area: "postgresql: query tuning"
tags: postgresql, sql, explain, performance
sources:
  - id: S1
    title: "PostgreSQL: Documentation: 18: 14.1. Using EXPLAIN"
    publisher: PostgreSQL Global Development Group
    url: https://www.postgresql.org/docs/current/using-explain.html
    accessed: 2026-11-04
    license: "PostgreSQL License"
  - id: S2
    title: "PostgreSQL: Documentation: 18: EXPLAIN"
    publisher: PostgreSQL Global Development Group
    url: https://www.postgresql.org/docs/current/sql-explain.html
    accessed: 2026-11-04
    license: "PostgreSQL License"
---
# Read a PostgreSQL query plan with EXPLAIN

## Short answer
Put `EXPLAIN` before a query to see the plan PostgreSQL picked: scan types, joins and cost estimates. `EXPLAIN ANALYZE` actually runs it and adds real timings, so wrap writes in a transaction you roll back. [S1][S2]

## Reading it
- Each node shows estimated start-up cost, total cost, rows and width. [S1]
- A Seq Scan with no WHERE clause is normal; on a filtered big table it may mean a missing index.
- Compare estimated and actual rows; big gaps suggest stale statistics, so run `ANALYZE`.
- Use `FORMAT JSON` for tools. [S2]

## Claims
- id: C1
  claim: EXPLAIN shows the planner's execution plan.
  source: S2
  evidence: "This command displays the execution plan that the PostgreSQL planner generates"
- id: C2
  claim: ANALYZE actually executes the statement.
  source: S2
  evidence: "The ANALYZE option causes the statement to be actually executed, not only planned."
- id: C3
  claim: A query with no WHERE clause scans all rows.
  source: S1
  evidence: "this query has no WHERE clause, it must scan all the rows"
- id: C4
  claim: Plan nodes show estimated start-up cost.
  source: S1
  evidence: "Estimated start-up cost."

## Sources
- [S1] PostgreSQL: Documentation: 18: 14.1. Using EXPLAIN. PostgreSQL Global Development Group. https://www.postgresql.org/docs/current/using-explain.html . License: PostgreSQL License. Accessed 2026-11-04.
- [S2] PostgreSQL: Documentation: 18: EXPLAIN. PostgreSQL Global Development Group. https://www.postgresql.org/docs/current/sql-explain.html . License: PostgreSQL License. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).