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
  #       ]
  #     ]
  #   ]
  # ]
