Execution example::

  select Entries \
    --limit -1 \
    --output_column _id,n_likes \
    --drilldown tag \
    --drilldown_calc_types AVG \
    --drilldown_calc_target n_likes \
    --drilldown_output_columns _key,_avg
  # [
  #   [
  #     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"
  #       ]
  #     ], 
  #     [
  #       [
  #         3
  #       ], 
  #       [
  #         [
  #           "_key", 
  #           "ShortText"
  #         ], 
  #         [
  #           "_avg", 
  #           "Float"
  #         ]
  #       ], 
  #       [
  #         "Hello", 
  #         5.0
  #       ], 
  #       [
  #         "Groonga", 
  #         12.5
  #       ], 
  #       [
  #         "Senna", 
  #         3.0
  #       ]
  #     ]
  #   ]
  # ]
