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

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

    if not lists.is_seq(exprs): exprs = [exprs]

    # Parse exprs

    old_exprs = exprs[:]

    exprs = []

    cols = []

    for i, expr in enumerate(old_exprs):

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

        expr = sql_gen.as_Col(expr)

        # Extract col

        if isinstance(expr, sql_gen.FunctionCall):

            col = expr.args[0]

            expr = sql_gen.Expr(expr)

        else: col = expr

        # Extract table

        if table == None:

            assert sql_gen.is_table_col(col)

            table = col.table

        col.table = None

        exprs.append(expr)

        cols.append(col)

    table = sql_gen.as_Table(table)

    index = sql_gen.Table(str(sql_gen.Col(','.join(map(str, cols)), table)))

    str_ = 'CREATE'

    if unique: str_ += ' UNIQUE'

    str_ += ' INDEX '+index.to_str(db)+' ON '+table.to_str(db)+' ('+(

        ', '.join((v.to_str(db) for v in exprs)))+')'

    try: run_query(db, str_, recover=True, cacheable=True, log_level=3)