mnesia的transform_table可以实现事务性的表结构转换，但是它本身有两个糟糕的地方：占用内存大、速度不够快。

先来看看mnesia:transform_table到底做了什么：

transform_table(Tab, Fun, NewAttrs, NewRecName)
  when is_function(Fun), is_list(NewAttrs), is_atom(NewRecName) ->
    schema_transaction(fun() -> do_transform_table(Tab, Fun, NewAttrs, NewRecName) end);

transform_table(Tab, ignore, NewAttrs, NewRecName)
  when is_list(NewAttrs), is_atom(NewRecName) ->
    schema_transaction(fun() -> do_transform_table(Tab, ignore, NewAttrs, NewRecName) end);

transform_table(Tab, Fun, NewAttrs, NewRecName) ->
    {aborted,{bad_type, Tab, Fun, NewAttrs, NewRecName}}.

schema_transaction(Fun) ->
    case get(mnesia_activity_state) of
	undefined ->
	    Args = [self(), Fun, whereis(mnesia_controller)],
	    Pid = spawn_link(?MODULE, schema_coordinator, Args),
	    receive
		{transaction_done, Res, Pid} -> Res;
		{'EXIT', Pid, R} -> {aborted, {transaction_crashed, R}}
	    end;
	_ ->
            {aborted, nested_transaction}
    end.

%% This process may dump the transaction log, and should
%% therefore not be run in an application process
%%
schema_coordinator(Client, _Fun, undefined) ->
    Res = {aborted, {node_not_running, node()}},
    Client ! {transaction_done, Res, self()},
    unlink(Client);

schema_coordinator(Client, Fun, Controller) when is_pid(Controller) ->
    %% Do not trap exit in order to automatically die
    %% when the controller dies

    link(Controller),
    unlink(Client),

    %% Fulfull the transaction even if the client dies
    Res = mnesia:transaction(Fun),
    Client ! {transaction_done, Res, self()},
    unlink(Controller),         % Avoids spurious exit message
    unlink(whereis(mnesia_tm)), % Avoids spurious exit message
    exit(normal).

看起来其实就是个transaction，不过同时做了schema的更新，这里有个坑后面会提到。总结下transform_table做的事情：
trans begin -> update schema -> copy records（write to list args） and update records -> trans end
既然如此，这里使用事务可以保证数据以及结构是同步更新的，而事务的过程中会拷贝至少一份数据（单节点），因此我们可以使用如下的思路来实现：

1. 保证更新数据时只有一个操作者（串行化）
2. trans begin -> update schema -> trans end -> update records

最终实现为：

%% @doc user-defined的transform
userdefined_transform() ->
    io:format("userdefined_transform begin time ~p~n",

), {atomic, ok} = mnesia:transform_table(db_test_transform, ignore, [k, v, v2]), [begin case mnesia:dirty_read(db_test_transform, K) of {test, K, V} -> mnesia:dirty_write(db_test_transform, {test, K, V, 0}); _ -> ok end end || K <- mnesia:dirty_all_keys(db_test_transform)], io:format("userdefined_transform end time ~p~n",

), ok.

详细代码在这里： https://github.com/qingliangcn/erlang_test

结果：

测试自定义transform_table => mnesia_userdefined_transform:test_userdefined_transform/0
delete table db_test_transform
create table db_test_transform
begin to insert test data 1000000
finish insert test data
userdefined_transform begin time {{2014,3,31},{20,19,22}}
userdefined_transform end time {{2014,3,31},{20,19,22}}

测试mnesia:transform_table => mnesia_userdefined_transform:test_mnesia_transform/0
delete table db_test_transform
create table db_test_transform
begin to insert test data 1000000
finish insert test data
mnesia_transform begin time {{2014,3,31},{20,19,33}}
mnesia_transform end time {{2014,3,31},{20,19,41}}