PostgreSQL: List databases with no current activity

unused_databases.sql

-- Show databases with no current connections
--
-- BIG FAT WARNING: Don't trust this blindly! Just because there's no *current*
-- activity does not mean a database is no longer needed.
--
SELECT
  (pg_stat_file ('base/' || d.oid || '/PG_VERSION')).modification,
  d.datname,
  pg_size_pretty(pg_database_size(d.datname)) AS size
FROM pg_database d
  LEFT JOIN pg_stat_activity a ON d.oid = a.datid
WHERE
  a.query IS NULL AND
  d.datname NOT LIKE 'template%'
ORDER BY datname;