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
  }
}
