Q: 加入一个新节点到Mnesia System
> mnesia:start().
> mnesia:change_config(extra_db_nodes, [some_existing_node]). %% disk-less node now
> mnesia:change_table_copy_type(schema, node(), disc_copies).
> mnesia:create_schema
> mneisa:delete_schema
> mnesia:add_table_copy
> mnesia:del_table_copy
1,在分布式系统中，获得共享表player的总记录数