Execution example::

  plugin_register functions/string
  # [[0, 1337566253.89858, 0.000355720520019531], true]
  select \
    --table Memos \
    --filter 'in_records(Conditions, \
                         user,                 "@", "user_pattern", \
                         tag,                 "==", "tag", \
                         string_length(_key), "<=", "max_length")' \
    --sort_by _id \
    --output_columns _key,user,tag
  # [
  #   [
  #     0, 
  #     1337566253.89858, 
  #     0.000355720520019531
  #   ], 
  #   [
  #     [
  #       [
  #         2
  #       ], 
  #       [
  #         [
  #           "_key", 
  #           "ShortText"
  #         ], 
  #         [
  #           "user", 
  #           "ShortText"
  #         ], 
  #         [
  #           "tag", 
  #           "Tags"
  #         ]
  #       ], 
  #       [
  #         "Groonga is fast", 
  #         "alice", 
  #         "groonga"
  #       ], 
  #       [
  #         "Droonga is fast", 
  #         "david", 
  #         "droonga"
  #       ]
  #     ]
  #   ]
  # ]
