Execution example::

  table_create NestedDrilldownTags TABLE_PAT_KEY ShortText
  # [[0, 1337566253.89858, 0.000355720520019531], true]
  column_create NestedDrilldownTags category COLUMN_SCALAR ShortText
  # [[0, 1337566253.89858, 0.000355720520019531], true]
  table_create NestedDrilldownMemos TABLE_HASH_KEY ShortText
  # [[0, 1337566253.89858, 0.000355720520019531], true]
  column_create NestedDrilldownMemos tag COLUMN_SCALAR NestedDrilldownTags
  # [[0, 1337566253.89858, 0.000355720520019531], true]
  load --table NestedDrilldownMemos
  [
  {"_key": "Groonga is fast!", "tag": "Groonga"},
  {"_key": "Groonga sticker!", "tag": "Groonga"},
  {"_key": "Mroonga sticker!", "tag": "Mroonga"},
  {"_key": "Rroonga is fast!", "tag": "Rroonga"}
  ]
  # [[0, 1337566253.89858, 0.000355720520019531], 4]
  load --table NestedDrilldownTags
  [
  {"_key": "Groonga", "category": "C/C++"},
  {"_key": "Mroonga", "category": "C/C++"},
  {"_key": "PGroonga", "category": "C/C++"},
  {"_key": "Rroonga", "category": "Ruby"}
  ]
  # [[0, 1337566253.89858, 0.000355720520019531], 4]
  select NestedDrilldownMemos \
    --drilldowns[Tag].keys tag \
    --drilldowns[Tag].output_columns _key \
    --drilldowns[Category].table Tag \
    --drilldowns[Category].keys category \
    --drilldowns[Category].output_columns _key,_nsubrecs
  # [
  #   [
  #     0, 
  #     1337566253.89858, 
  #     0.000355720520019531
  #   ], 
  #   [
  #     [
  #       [
  #         4
  #       ], 
  #       [
  #         [
  #           "_id", 
  #           "UInt32"
  #         ], 
  #         [
  #           "_key", 
  #           "ShortText"
  #         ], 
  #         [
  #           "tag", 
  #           "NestedDrilldownTags"
  #         ]
  #       ], 
  #       [
  #         1, 
  #         "Groonga is fast!", 
  #         "Groonga"
  #       ], 
  #       [
  #         2, 
  #         "Groonga sticker!", 
  #         "Groonga"
  #       ], 
  #       [
  #         3, 
  #         "Mroonga sticker!", 
  #         "Mroonga"
  #       ], 
  #       [
  #         4, 
  #         "Rroonga is fast!", 
  #         "Rroonga"
  #       ]
  #     ], 
  #     {
  #       "Category": [
  #         [
  #           2
  #         ], 
  #         [
  #           [
  #             "_key", 
  #             "ShortText"
  #           ], 
  #           [
  #             "_nsubrecs", 
  #             "Int32"
  #           ]
  #         ], 
  #         [
  #           "C/C++", 
  #           2
  #         ], 
  #         [
  #           "Ruby", 
  #           1
  #         ]
  #       ], 
  #       "Tag": [
  #         [
  #           3
  #         ], 
  #         [
  #           [
  #             "_key", 
  #             "ShortText"
  #           ]
  #         ], 
  #         [
  #           "Groonga"
  #         ], 
  #         [
  #           "Mroonga"
  #         ], 
  #         [
  #           "Rroonga"
  #         ]
  #       ]
  #     }
  #   ]
  # ]
