Cookbook recipe

Stop disk-spilled sorts by tuning work_mem

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

EXPLAIN shows "Sort Method: external merge Disk". The sort did not fit in memory and spilled to disk, making it slow. Diagnose it Look for external sorts/hashes: EXPLAIN (ANALYZE) SELECT ... ORDER BY ...; -- Sort…

Investigation Path

EXPLAIN shows “Sort Method: external merge Disk”. The sort did not fit in memory and spilled to disk, making it slow.

Diagnose it

Look for external sorts/hashes:

EXPLAIN (ANALYZE)
SELECT ... ORDER BY ...;
-- Sort Method: external merge  Disk: 124032kB

Why it happens

Each sort/hash node gets up to work_mem. When the data exceeds it, PostgreSQL spills to temp files on disk. Too-low work_mem causes spills; too-high risks OOM because every node and connection can use it.

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

CREATE INDEX CONCURRENTLY: Building Indexes Online Disk-Based Sort Spill Connection Exhaustion WAL Archive Failure Tablespace Disk Full
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