Execution example::

  logical_select \
    --logical_table Entries \
    --shard_key created_at \
    --limit 0 \
    --output_columns _id \
    --drilldowns[tag].keys tag \
    --drilldowns[tag].columns[nsubrecs_string].stage initial \
    --drilldowns[tag].columns[nsubrecs_string].type ShortText \
    --drilldowns[tag].columns[nsubrecs_string].value _nsubrecs \
    --drilldowns[tag].output_columns _key,_nsubrecs,nsubrecs_string
  # [
  #   [
  #     0,
  #     1337566253.89858,
  #     0.000355720520019531
  #   ],
  #   [
  #     [
  #       [
  #         5
  #       ],
  #       [
  #         [
  #           "_id",
  #           "UInt32"
  #         ]
  #       ]
  #     ],
  #     {
  #       "tag": [
  #         [
  #           3
  #         ],
  #         [
  #           [
  #             "_key",
  #             "ShortText"
  #           ],
  #           [
  #             "_nsubrecs",
  #             "Int32"
  #           ],
  #           [
  #             "nsubrecs_string",
  #             "ShortText"
  #           ]
  #         ],
  #         [
  #           "Hello",
  #           1,
  #           "1"
  #         ],
  #         [
  #           "Groonga",
  #           2,
  #           "2"
  #         ],
  #         [
  #           "Senna",
  #           2,
  #           "2"
  #         ]
  #       ]
  #     }
  #   ]
  # ]
