def clear_batch *types
    types = [:insert, :update, :delete] if types.empty?
    types.each do |type|
      raise ArgumentError.new("Invalid type: #{type}") unless @pstmts.has_key?(type)
      @pstmts[type].values.each(&:clear_batch)
    end
    nil
  end
def clear_batch *types
    types = [:insert, :update, :delete] if types.empty?
    types.each do |type|
      raise ArgumentError.new("Invalid type: #{type}") unless @pstmts.has_key?(type)
      @pstmts[type].values.each(&:clear_batch)
    end
    nil
  end
Clear batched operations.
 @param [*Symbol] types Types of batched operations to clear.
   If not given, :insert, :update and :delete.
 @return [nil]