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>", 
  #     [
  #       [
  #         "Groonga::Sharding::LogicalSelectCommand::Executor.execute_search", 
  #         "/home/horimoto/work/release/groonga/groonga.clean/plugins/sharding/logical_select.rb", 
  #         560
  #       ]
  #     ]
  #   ]
  # ]
