PostgreSQL Vacuuming

Vacuuming

The vacuuming process in PostgreSQL is an essential maintenance operation that helps optimize database performance and manage storage. Here are the key aspects of PostgreSQL’s vacuuming process:

1. Purpose:

   • Reclaims storage occupied by dead tuples (rows that have been deleted or updated)¹²
   • Prevents transaction ID wraparound issues²
   • Updates statistics for the query planner²
   • Improves overall query performance³

2. Types of VACUUM:

   • Standard VACUUM: Reclaims space and makes it available for reuse within the same table¹
   • VACUUM FULL: Rewrites the entire table, reclaiming more space but requiring an exclusive lock and more time[1][3]

3. Autovacuum:

   • PostgreSQL includes an autovacuum daemon that automatically performs VACUUM operations²
   • It schedules vacuuming dynamically based on update activity²

4. How it works:

   • Scans tables to remove dead tuples resulting from UPDATE and DELETE operations³
   • Marks space occupied by dead tuples as reusable by other tuples³
   • Updates system catalogs with current statistics³

5. Concurrent operation:

   • Standard VACUUM can run in parallel with normal database operations (SELECT, INSERT, UPDATE, DELETE)¹
   • VACUUM FULL requires an ACCESS EXCLUSIVE lock and cannot run concurrently with other table operations¹

6. Performance considerations:

   • VACUUM creates I/O traffic, which can impact performance of other active sessions²
   • Configuration parameters can be adjusted to reduce the performance impact of background vacuuming²

7. Frequency:

   • Regular vacuuming is necessary, especially for frequently updated tables¹
   • The frequency depends on the database’s update rate and available resources²

8. Best practices:

   • Use autovacuum for most situations, adjusting its parameters as needed²
   • Schedule manual VACUUMs during low-usage periods if necessary²
   • Use VACUUM FULL sparingly, only when significant space needs to be reclaimed²

9. Monitoring:

   • It’s important to monitor VACUUM processes to ensure they’re running efficiently⁴
   • You can query pg_stat_user_tables to check when tables were last vacuumed⁴

By regularly performing VACUUM operations, either through autovacuum or manual scheduling, PostgreSQL can maintain optimal performance, manage disk space effectively, and prevent potential issues related to transaction ID wraparound.

Vacuuming progress

Aside from executing a VACUUM VERBOSE, vacuuming process, which will output the different steps it is executing, this approach can approximate the level of progress of the vacuuming process.⁵

SELECT
	heap_blks_scanned/CAST(heap_blks_total AS NUMERIC) * 100 AS heap_blks_percent,
	progress.*,
	activity.query
FROM pg_stat_progress_vacuum AS progress
INNER JOIN pg_stat_activity AS activity ON activity.pid = progress.pid;

Footnotes

1. https://www.postgresql.org/docs/current/sql-vacuum.html ↩ ↩² ↩³ ↩⁴ ↩⁵

2. https://www.postgresql.org/docs/current/routine-vacuuming.html ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹

3. https://www.enterprisedb.com/blog/postgresql-vacuum-and-analyze-best-practice-tips ↩ ↩² ↩³ ↩⁴

4. https://www.datadoghq.com/blog/postgresql-vacuum-monitoring/ ↩ ↩²

5. https://dba.stackexchange.com/a/245163/2704 ↩