add_index_options

# File activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb, line 774
        def add_index_options(table_name, column_name, options = {})
          column_names = Array(column_name)
          index_name   = index_name(table_name, column: column_names)

          if Hash === options # legacy support, since this param was a string
            options.assert_valid_keys(:unique, :order, :name, :where, :length, :internal, :using, :algorithm, :type)

            index_type = options[:unique] ? "UNIQUE" : ""
            index_type = options[:type].to_s if options.key?(:type)
            index_name = options[:name].to_s if options.key?(:name)
            max_index_length = options.fetch(:internal, false) ? index_name_length : allowed_index_name_length

            if options.key?(:algorithm)
              algorithm = index_algorithms.fetch(options[:algorithm]) {
                raise ArgumentError.new("Algorithm must be one of the following: #{index_algorithms.keys.map(&:inspect).join(', ')}")
              }
            end

            using = "USING #{options[:using]}" if options[:using].present?

            if supports_partial_index?
              index_options = options[:where] ? " WHERE #{options[:where]}" : ""
            end
          else
            if options
              message = "Passing a string as third argument of `add_index` is deprecated and will" +
                " be removed in Rails 4.1." +
                " Use add_index(#{table_name.inspect}, #{column_name.inspect}, unique: true) instead"

              ActiveSupport::Deprecation.warn message
            end

            index_type = options
            max_index_length = allowed_index_name_length
            algorithm = using = nil
          end

          if index_name.length > max_index_length
            raise ArgumentError, "Index name '#{index_name}' on table '#{table_name}' is too long; the limit is #{max_index_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
          index_columns = quoted_columns_for_index(column_names, options).join(", ")

          [index_name, index_type, index_columns, index_options, algorithm, using]
        end