def add_index(db, exprs, table=None, unique=False, ensure_not_null_=True):

    '''Adds an index on column(s) or expression(s) if it doesn't already exist.

    @param ensure_not_null_ If set, translates NULL values to sentinel values.

        This allows indexes to be used for comparisons where NULLs are equal.

    '''

        # Extract col

        expr = copy.deepcopy(expr) # don't modify input!

            expr = sql_gen.Expr(expr)

        # Extract table

        if table == None:

            assert sql_gen.is_table_col(col)

            table = col.table

        exprs.append(expr)

        cols.append(col)