def db_type
      # first, detect any character not number
      case
      when @data.any? { |v| v.to_s =~ DATE_REGEXP }
        'DATE'
      when @data.any? { |v| v.to_s =~ /[^0-9e.-]/ }
        'VARCHAR (255)'
      when @data.any? { |v| v.to_s =~ /\./ }
        'DOUBLE'
      else
        'INTEGER'
      end
    end
Returns the database type for the vector, according to its content