Execution example::

  plugin_register functions/string
  # [[0,1337566253.89858,0.000355720520019531],true]
  select Entries \
    --columns[content_length].stage initial \
    --columns[content_length].type UInt32 \
    --columns[content_length].value 'string_length(content)' \
    --output_columns _id,content,content_length
  # [
  #   [
  #     0,
  #     1337566253.89858,
  #     0.000355720520019531
  #   ],
  #   [
  #     [
  #       [
  #         5
  #       ],
  #       [
  #         [
  #           "_id",
  #           "UInt32"
  #         ],
  #         [
  #           "content",
  #           "Text"
  #         ],
  #         [
  #           "content_length",
  #           "UInt32"
  #         ]
  #       ],
  #       [
  #         1,
  #         "Welcome! This is my first post!",
  #         31
  #       ],
  #       [
  #         2,
  #         "I started to use Groonga. It's very fast!",
  #         41
  #       ],
  #       [
  #         3,
  #         "I also started to use Mroonga. It's also very fast! Really fast!",
  #         64
  #       ],
  #       [
  #         4,
  #         "I migrated all Senna system!",
  #         28
  #       ],
  #       [
  #         5,
  #         "I also migrated all Tritonn system!",
  #         35
  #       ]
  #     ]
  #   ]
  # ]
