Execution example::

  select Entries \
    --drilldowns[tag].keys tag \
    --drilldowns[tag].columns[is_popular].stage initial \
    --drilldowns[tag].columns[is_popular].type Bool \
    --drilldowns[tag].columns[is_popular].value '_nsubrecs > 1' \
    --drilldowns[tag].filter is_popular \
    --drilldowns[tag].output_columns _key,is_popular,_nsubrecs
  # [
  #   [
  #     0,
  #     1337566253.89858,
  #     0.000355720520019531
  #   ],
  #   [
  #     [
  #       [
  #         5
  #       ],
  #       [
  #         [
  #           "_id",
  #           "UInt32"
  #         ],
  #         [
  #           "_key",
  #           "ShortText"
  #         ],
  #         [
  #           "content",
  #           "Text"
  #         ],
  #         [
  #           "n_likes",
  #           "UInt32"
  #         ],
  #         [
  #           "tag",
  #           "ShortText"
  #         ]
  #       ],
  #       [
  #         1,
  #         "The first post!",
  #         "Welcome! This is my first post!",
  #         5,
  #         "Hello"
  #       ],
  #       [
  #         2,
  #         "Groonga",
  #         "I started to use Groonga. It's very fast!",
  #         10,
  #         "Groonga"
  #       ],
  #       [
  #         3,
  #         "Mroonga",
  #         "I also started to use Mroonga. It's also very fast! Really fast!",
  #         15,
  #         "Groonga"
  #       ],
  #       [
  #         4,
  #         "Good-bye Senna",
  #         "I migrated all Senna system!",
  #         3,
  #         "Senna"
  #       ],
  #       [
  #         5,
  #         "Good-bye Tritonn",
  #         "I also migrated all Tritonn system!",
  #         3,
  #         "Senna"
  #       ]
  #     ],
  #     {
  #       "tag": [
  #         [
  #           2
  #         ],
  #         [
  #           [
  #             "_key",
  #             "ShortText"
  #           ],
  #           [
  #             "is_popular",
  #             "Bool"
  #           ],
  #           [
  #             "_nsubrecs",
  #             "Int32"
  #           ]
  #         ],
  #         [
  #           "Groonga",
  #           true,
  #           2
  #         ],
  #         [
  #           "Senna",
  #           true,
  #           2
  #         ]
  #       ]
  #     }
  #   ]
  # ]
