columns {
  name: "key"
  type {
    type_id: UINT32
  }
}
columns {
  name: "column1"
  type {
    optional_type {
      item {
        type_id: DOUBLE
      }
    }
  }
}
columns {
  name: "column2"
  type {
    type_id: DOUBLE
  }
}
columns {
  name: "column3"
  type {
    type_id: DOUBLE
  }
}
columns {
  name: "column4"
  type {
    optional_type {
      item {
        type_id: DOUBLE
      }
    }
  }
}
rows {
  items {
    uint32_value: 10
  }
  items {
    double_value: 1
  }
  items {
    double_value: 3
  }
  items {
    double_value: 1
  }
  items {
    null_flag_value: NULL_VALUE
  }
}
rows {
  items {
    uint32_value: 20
  }
  items {
    double_value: 0
  }
  items {
    double_value: 4.5
  }
  items {
    double_value: 1
  }
  items {
    null_flag_value: NULL_VALUE
  }
}
