Mnesia迭代:提供一组函数来迭代表里的所有记录,firecat实践

1、Mnesia提供了如下几个函数遍历所有记录


mnesia:foldl(Fun, Acc0, Tab) -> NewAcc | transaction abort
     mnesia:foldr(Fun, Acc0, Tab) -> NewAcc | transaction abort
     mnesia:foldl(Fun, Acc0, Tab, LockType) -> NewAcc | transaction abort
     mnesia:foldr(Fun, Acc0, Tab, LockType) -> NewAcc | transaction abort

这些函数会将Fun遍历应用到表Tab表上,并把结构放入累计集Acc0中,可以按需要指定锁类型。Fun有两个参数,第一个是从表中取出的记录,第二个是累计集。


例如要查找薪资级别在10以下的员工:

find_low_salaries() ->
     Constraint = 
          fun(Emp, Acc) when Emp#employee.salary < 10 ->
                 [Emp | Acc];
               (_, Acc) ->
                    Acc
             end,
        Find = fun() -> mnesia:foldl(Constraint, [], employee) end,
        mnesia:transaction(Find)


将薪资上调到10级,返回所有涨薪和:

increase_low_salaries() ->
         Increase = 
             fun(Emp, Acc) when Emp#employee.salary < 10 ->
                    OldS = Emp#employee.salary,
                    ok = mnesia:write(Emp#employee{salary = 10}),
                    Acc + 10 - OldS;
                (_, Acc) ->
                    Acc
             end,
        IncLow = fun() -> mnesia:foldl(Increase, 0, employee, write) end,
        mnesia:transaction(IncLow).

   

在遍历中可以做很多事情,但是要特别留意性能和内存消耗。



2、firecat的实践一,查询数据库表的某一列,并求和

get_onlinenum(<<"ALL_DEVICES">>) -> Mnesia迭代求和
    Sum =
    fun(Emp, Acc) when Emp#?ONLINE_TAB.onlinenum /= 0 ->
    V = Emp#?ONLINE_TAB.onlinenum,
        Acc + V;
    (_, Acc) ->
    Acc
    end,
    Find = fun() -> mnesia:foldl(Sum, 0, ?ONLINE_TAB) end,
    mnesia:transaction(Find);
(test@192.168.83.128)1> test:get_onlinenum(<<"ALL_DEVICES">>).
{atomic,2}


其中,{atomic,2}里面的2就是返回结果。


实践二,修改数据


clear_onlinenum() ->
    Clear =
    fun(Emp, Acc) when Emp#?ONLINE_TAB.onlinenum /= 0 ->
    ok = mnesia:write(Emp#?ONLINE_TAB{onlinenum = 0});
    (_, Acc) ->
    Acc
    end,
    Find = fun() -> mnesia:foldl(Clear, [], ?ONLINE_TAB) end,
    ret(mnesia:transaction(Find)).


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