'Find all columns of the primary key of the specified table'

def _find_primary_key_columns(self, table_name):

    db_name = self._get_setting('NAME')
    primary_key_columns = set()
    for (col, constraints) in self.lookup_constraint(db_name, table_name):
        for (kind, cname) in constraints:
            if (kind == 'PRIMARY    KEY'):
                primary_key_columns.add(col.lower())
    return primary_key_columns
