Issue
In the Heroku Dashboard, my Heroku Postgres database shows commit activity, in the graphs section, even when the database is not being used.
Resolution
This is expected behaviour. Postgres treats each executed SQL statement as being implicitly wrapped in a transaction, so for example a SELECT 1; query is treated as BEGIN; SELECT 1; COMMIT;.
As a result, each time a statement is executed successfully, xact_commit is incremented in pg_stat_database, which is what Heroku Postgres monitoring uses to determine the commit activity. Our monitoring tooling itself periodically executes queries to check the health of your database, which causes a baseline level of commits being recorded for your database.