Cookbook recipe

Diagnose detoasting overhead on wide rows

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

What This Is

A production experience capsule based on real incident patterns.

Why It Matters

Builds production judgment under pressure.

Next Step

Run the decision point → · Enter full simulator →

Estimated investigation4 min

Scenario

Selecting a few small columns is fast, but adding a big text/jsonb column makes the query crawl. TOAST detoasting is the cost. Diagnose it Check column storage and TOAST size: SELECT pg_size_pretty(pg_relation_size(reltoastrelid)) AS toast_size FROM pg_class…

Investigation Path

Selecting a few small columns is fast, but adding a big text/jsonb column makes the query crawl. TOAST detoasting is the cost.

Diagnose it

Check column storage and TOAST size:

SELECT pg_size_pretty(pg_relation_size(reltoastrelid)) AS toast_size
FROM pg_class WHERE relname = 'docs';

Why it happens

Large values are compressed and stored out-of-line in a TOAST table. Reading them means extra I/O and decompression. SELECT * pulls and detoasts those values even when you do not need them.

This is a Pro lesson

Get every Learning Pathway and cookbook recipe — grounded in PostgreSQL source code, with diagnostics, fixes, and prevention for each topic.

Continue this lesson to learn:

  • How to fix it
  • Prevent it next time
  • Related & next steps
  • All 36 Learning Pathway lessons
  • 170+ cookbook recipes
  • Source-grounded diagnostics & fixes
Unlock everything — $24.99/month $199/year — save 35%

Secure checkout Cancel anytime Source-grounded

Career Impact

This scenario builds production judgment and operational confidence under pressure.

Open Career Dashboard →

Keep going

Related & next steps

More like this

Detecting and stopping a runaway query Password Authentication Failed AWS Aurora PostgreSQL: The Log-Is-the-Database Storage Engine What ANALYZE collects and why plans depend on it WAL Disk Full

Concepts on this page

TOAST
Browse the cookbook Step-by-step recipes Follow a pathway Learn it in order Open the glossary Every concept, plainly
Was this helpful?

← All cookbook recipes