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
  #         ]
  #       ]
  #     }
  #   ]
  # ]
