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: "c.key"
  type {
    type_id: STRING
  }
}
columns {
  name: "c.value"
  type {
    type_id: STRING
  }
}
