distinct_on_all = object() # tells mk_select() to SELECT DISTINCT ON all columns

    start=None, order_by=order_by_pkey, table_is_esc=False, default_table=None):

    '''

    @param tables The single table to select from, or a list of tables to join

    query = 'SELECT'

    # DISTINCT ON columns

        query += ' DISTINCT'

        if distinct_on != distinct_on_all:

            query += ' ON ('+(', '.join(map(parse_col, distinct_on)))+')'

            pkeys_table_exists_ref[0] = True

    conds = {}

    def mk_main_select(cols):

        return mk_select(db, insert_joins, cols, conds, distinct_on,

            order_by=None, limit=limit, start=start, table_is_esc=table_is_esc,