columns {
  name: "a.k2"
  type {
    type_id: STRING
  }
}
columns {
  name: "a.u2"
  type {
    type_id: STRING
  }
}
columns {
  name: "a.v2"
  type {
    type_id: STRING
  }
}
columns {
  name: "b.k1"
  type {
    type_id: STRING
  }
}
columns {
  name: "b.u1"
  type {
    type_id: STRING
  }
}
columns {
  name: "b.v1"
  type {
    type_id: STRING
  }
}
