Ensure table's existence - as per the gludb spec.

def ensure_table(self, cls):
        """Ensure table's existence - as per the gludb spec."""
        id_len = len(uuid())
        index_names = cls.index_names() or []
        cols = [
            'id char(%d) primary key' % (id_len,),
            'value jsonb'
        ] + [
            name + ' text' for name in index_names
        ]

        table_name = cls.get_table_name()

        with self._conn() as conn:
            with conn.cursor() as cur:
                cur.execute('create table if not exists %s (%s);' % (
                    table_name,
                    ','.join(cols)
                ))
                for name in index_names:
                    cur.execute('create index if not exists %s on %s(%s);' % (
                        table_name + '_' + name + '_idx',
                        table_name,
                        name
                    ))