code: func NewPostgresMigrator(db *sql.DB) *Migrator {
  	m := NewMigrator(db)
  	m.Locker = newPostgresLocker(db)
  	return m
  }
docs: // NewPostgresMigrator returns a new Migrator instance that uses the underlying
  // sql.DB connection to a postgres database to perform migrations. It will use
  // Postgres's advisory locks to ensure that only 1 migration is run at a time.