Add an entry about VACUUM FULL to Maintenance docs (#3348)

docs/admin/maintenance.rst

@@ -54,3 +54,42 @@ and add them to the BLOCKED_IPs config variable in Heroku Settings.
 
 .. _DDoS: https://en.wikipedia.org/wiki/Denial-of-service_attack
 .. _IP detection script: https://github.com/mozilla-l10n/pontoon-scripts/blob/main/dev/check_ips_heroku_log.py
+
+Vacuuming a Database
+--------------------
+To reduce the size of Postgres DB tables and improve performance, it is recommended to
+`vacuum the database`_ regularly. Heroku already does that partially by running the
+`VACUUM` command automatically, but that only marks the space as available for reuse.
+
+Running `VACUUM FULL` offers a more exhaustive cleanup and reduces bloat.
+
+   .. Warning::
+
+    `VACUUM FULL` is a heavyweight operation, which prevents any other statements from
+    running concurrently, even simple SELECT queries. For most tables it only takes a
+    few seconds to complete, but on the bigger tables it can take up to a few minutes.
+    During that time, the application will be unresponsive.
+
+You can run `VACUUM FULL` with the following command:
+
+.. code-block:: bash
+
+   $ heroku pg:psql --app mozilla-pontoon
+   => VACUUM FULL table_name;
+
+To list the DB tables, ordered by size, run:
+
+.. code-block:: bash
+
+   $ heroku pg:psql --app mozilla-pontoon
+   => SELECT
+        table_name,
+        pg_size_pretty(pg_total_relation_size(table_name::text)) AS size
+      FROM
+        information_schema.tables
+      WHERE
+        table_schema = 'public'
+      ORDER BY
+        pg_total_relation_size(table_name::text) DESC;
+
+.. _vacuum the database: https://devcenter.heroku.com/articles/managing-vacuum-on-heroku-postgres