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PostgreSQL autovacuum not triggering on large high-churn table causing query slowdown

Fresh14 days ago

Mar 14, 20260 views

Confidence Score68%

68%

Problem

Queries on a heavily updated PostgreSQL table degrade over time despite indexes being present. EXPLAIN ANALYZE shows sequential scans. pg_stat_user_tables shows n_dead_tup in the millions and last_autovacuum is hours or days ago. The default autovacuum_vacuum_scale_factor of 0.2 (20%) means autovacuum waits until 20% of the table is dead tuples — on a 50M row table that is 10M dead tuples before autovacuum triggers.

Error Output

Seq Scan on orders  (cost=0.00..4523432.00 rows=8234521 width=312) (actual time=0.023..45234.432 rows=8234521 loops=1)

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1 Fix

Canonical Fix

Moderate Confidence Fix

66% confidence78% success rate5 verificationsLast verified Mar 14, 2026

Tune autovacuum settings per-table and run VACUUM ANALYZE manually

Medium Risk

The default scale_factor of 0.2 (20%) means autovacuum only triggers after 20% of rows are dead. On large tables this can be millions of rows. Lowering the threshold per-table makes autovacuum run more frequently.

Trust Score

5 verifications

78% success

Check dead tuple count

Identify the bloated table:

sql

SELECT relname, n_dead_tup, n_live_tup, last_autovacuum
FROM pg_stat_user_tables
ORDER BY n_dead_tup DESC LIMIT 10;

2
Run VACUUM ANALYZE immediately
Clean up existing dead tuples now:
sql
```
VACUUM ANALYZE your_table_name;
```

Set aggressive autovacuum for high-churn tables

Lower the scale factor so autovacuum triggers earlier:

sql

ALTER TABLE your_table_name SET (
  autovacuum_vacuum_scale_factor = 0.01,
  autovacuum_analyze_scale_factor = 0.005,
  autovacuum_vacuum_cost_delay = 2
);

Validation

EXPLAIN ANALYZE shows Index Scan instead of Seq Scan after vacuum. Query time improves significantly.

Verification Summary

Worked: 5

Partial: 2

Failed: 2

Last verified Mar 14, 2026

Environment

Product: PostgreSQL
Version: 15
Environment: production

Submitted by

Alex Chen

2450 rep