code: def sql_index(self,index_name,column_names,unique=True):
          
          if type(column_names) == str:
              column_names = [column_names]
          try:
              if len(column_names) == 0:
                  raise TypeError
          except TypeError:
              raise ValueError("Provide a list of column names for an index.")
          if unique:
              UNIQUE = "UNIQUE"
          else:
              UNIQUE = ""
          table_name = self.name
          columns = ",".join(column_names)
          SQL = """CREATE %(UNIQUE)s INDEX %(index_name)s ON %(table_name)s """\
              """(%(columns)s)""" % locals()
          self.sql(SQL)
docs: Add a named index on given columns to improve performance.