columns {
  name: "A.key"
  type {
    type_id: STRING
  }
}
columns {
  name: "A.subkey"
  type {
    type_id: STRING
  }
}
columns {
  name: "A.value"
  type {
    type_id: STRING
  }
}
columns {
  name: "B.key"
  type {
    type_id: STRING
  }
}
columns {
  name: "B.subkey"
  type {
    type_id: STRING
  }
}
columns {
  name: "B.value"
  type {
    type_id: STRING
  }
}
