Execution example::

  select Memos \
    --match_columns "title || scorer_tf_at_most(content, 2.0)" \
    --query "Groonga" \
    --output_columns "title, content, _score" \
    --sort_keys "-_score"
  # [
  #   [
  #     0, 
  #     1337566253.89858, 
  #     0.000355720520019531
  #   ], 
  #   [
  #     [
  #       [
  #         2
  #       ], 
  #       [
  #         [
  #           "title", 
  #           "ShortText"
  #         ], 
  #         [
  #           "content", 
  #           "Text"
  #         ], 
  #         [
  #           "_score", 
  #           "Int32"
  #         ]
  #       ], 
  #       [
  #         "Groonga is fast", 
  #         "Groonga! Groonga! Groonga! Groonga is very fast!", 
  #         3
  #       ], 
  #       [
  #         "Groonga is easy", 
  #         "Groonga is very easy full text search engine!", 
  #         2
  #       ]
  #     ]
  #   ]
  # ]
