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"
  #         ]
  #       ]
  #     }
  #   ]
  # ]
