def get_existing_keys(self, events):
        """Returns the list of keys from the given event source that are already in the DB"""
        data = [e[self.key] for e in events]
        ss = ','.join(['%s' for _ in data])
        query = 'SELECT %s FROM %s WHERE %s IN (%s)' % (self.key, self.table, self.key, ss)
        cursor = self.conn.conn.cursor()
        cursor.execute(query, data)
        LOG.info("%s (data: %s)", query, data)
        existing = [r[0] for r in cursor.fetchall()]
        LOG.info("Existing IDs: %s" % existing)
        return set(existing)
Returns the list of keys from the given event source that are already in the DB