PostgreSQL does not overwrite rows in place. Every UPDATE/DELETE creates dead tuples that must be cleaned up. VACUUM is the mechanism that reclaims space, keeps visibility maps current, and prevents transaction ID wraparound. When tables grow large in Rails apps, vacuum strategy becomes critical for stability and performance.

Why vacuum matters in large Rails tables

Large tables (events, logs, audit, payments) tend to:

• accumulate dead tuples quickly due to frequent updates
• cause bloated indexes that slow down reads and writes
• increase I/O for sequential scans
• threaten transaction ID wraparound if left unchecked

Autovacuum usually handles this, but defaults are conservative. For very large tables, you often need targeted tuning and occasional manual vacuum.

Vacuum types in short

• VACUUM: marks dead tuples as reusable and updates stats. Does not rewrite the table. Safe to run online.
• VACUUM (ANALYZE): also updates planner statistics immediately.
• VACUUM FULL: rewrites the table to reclaim space to the OS. Requires exclusive lock. Use rarely and during a maintenance window.

Rails-friendly approach for large tables

1. Tune autovacuum per table

For large, high-churn tables, set a lower threshold and scale factor so autovacuum runs more often on those tables only.

ALTER TABLE public.events SET (
  autovacuum_vacuum_scale_factor = 0.02,
  autovacuum_vacuum_threshold = 5000,
  autovacuum_analyze_scale_factor = 0.01,
  autovacuum_analyze_threshold = 5000
);

Tip: lower scale factor, higher threshold is a common pattern. Adjust based on row churn and available I/O.

2. Use manual VACUUM (ANALYZE) during off-peak

Schedule manual vacuum for the biggest tables if you see bloat or stale stats. This keeps the planner healthy and helps index usage.

VACUUM (ANALYZE, VERBOSE) public.events;

You can run this from a maintenance job or via a cron container that has access to the database.

3. Avoid VACUUM FULL in production

VACUUM FULL takes an exclusive lock and rewrites the table. It is rarely safe for large production tables in Rails apps. Prefer:

• regular VACUUM (ANALYZE)
• index reindexing in isolation
• table partitioning for time-series data

Rails migration for per-table settings

You can set per-table autovacuum settings using a migration. This is repeatable and versioned.

class TuneAutovacuumForEvents < ActiveRecord::Migration[7.1]
  def up
    execute <<~SQL
      ALTER TABLE public.events SET (
        autovacuum_vacuum_scale_factor = 0.02,
        autovacuum_vacuum_threshold = 5000,
        autovacuum_analyze_scale_factor = 0.01,
        autovacuum_analyze_threshold = 5000
      );
    SQL
  end

  def down
    execute <<~SQL
      ALTER TABLE public.events RESET (
        autovacuum_vacuum_scale_factor,
        autovacuum_vacuum_threshold,
        autovacuum_analyze_scale_factor,
        autovacuum_analyze_threshold
      );
    SQL
  end
end

Monitoring: what to check

Use these queries to understand bloat and vacuum progress.

SELECT
  relname,
  n_live_tup,
  n_dead_tup,
  last_vacuum,
  last_autovacuum,
  last_analyze,
  last_autoanalyze
FROM pg_stat_all_tables
WHERE schemaname = 'public'
ORDER BY n_dead_tup DESC
LIMIT 20;

If n_dead_tup stays high, autovacuum is falling behind. Consider:

• increasing autovacuum worker count
• increasing autovacuum cost limit
• lowering scale factor for that table

Large tables and Rails best practices

• Prefer append-only tables when possible.
• Use soft deletes sparingly on very large tables.
• Partition time-series tables to reduce vacuum scope.
• Keep btree indexes lean; remove unused indexes.
• Reindex in a controlled maintenance window if needed.

Summary

Vacuum keeps PostgreSQL healthy by reclaiming space and keeping statistics current. In Rails apps with large tables, per-table autovacuum tuning and scheduled VACUUM (ANALYZE) provide the best balance of safety and performance. Reserve VACUUM FULL for rare, offline maintenance.