PostgreSQL work_mem and Sort Spill

Performance / Guide 37 of 47 / Advanced

• OSUbuntu 26.04 LTS (resolute)
• PostgreSQL18.6-0ubuntu0.26.04.1
• Test data400,000 rows, 2 core / 4 GB host
• TimeAbout 16 min
• Reviewed27 August 2026

Written against the versions above. `work_mem` is per sort or hash **per node per worker**, not per query. A parallel query with several sorts can use many multiples of it, which is why raising it globally is riskier than it looks.

Before you start

• The events table with 400,000 rows.

1. 1

   Sort more than work_mem can hold

2. 2

   Raise work_mem and measure again

3. 3

   Find spills across the whole database

Official sources

• PostgreSQL 18 Documentation - Using EXPLAIN
• PostgreSQL 18 Documentation - Index Types
• PostgreSQL 18 Documentation - Partial Indexes
• PostgreSQL 18 Documentation - Routine Vacuuming
• PostgreSQL 18 Documentation - pg_stat_statements