# File lib/active_record/connection_adapters/abstract/schema_statements.rb, line 329
      def add_index(table_name, column_name, options = {})
        column_names = Array.wrap(column_name)
        index_name   = index_name(table_name, :column => column_names)

        if Hash === options # legacy support, since this param was a string
          index_type = options[:unique] ? "UNIQUE" : ""
          index_name = options[:name].to_s if options.key?(:name)
        else
          index_type = options
        end

        if index_name.length > index_name_length
          raise ArgumentError, "Index name '#{index_name}' on table '#{table_name}' is too long; the limit is #{index_name_length} characters"
        end
        if index_name_exists?(table_name, index_name, false)
          raise ArgumentError, "Index name '#{index_name}' on table '#{table_name}' already exists"
        end
        quoted_column_names = quoted_columns_for_index(column_names, options).join(", ")

        execute "CREATE #{index_type} INDEX #{quote_column_name(index_name)} ON #{quote_table_name(table_name)} (#{quoted_column_names})"
      end