Execution example::

  logical_select --logical_table Nonexistent --shard_key created_at
  # [
  #   [
  #     -22,
  #     1337566253.89858,
  #     0.000355720520019531,
  #     "[logical_select] no shard exists: logical_table: <Nonexistent>: shard_key: <created_at>",
  #     [
  #       [
  #         "execute_search",
  #         "lib/groonga/plugins/sharding/logical_select.rb",
  #         2929
  #       ]
  #     ]
  #   ]
  # ]
