如何使用mnesia:select/4和mnesia:select/1进行分页查询
如何使用mnesia:select/4和mnesia:select/1进行分页查询
提问于 2013-08-13 09:42:37
一个名为" md“的表具有结构{id，name}，我希望从md读取记录使用分页查询，我尝试了mnesia:select/4和mnesia:select/1，如下所示：
%% first use select/2: "ID < 10",returned [1,2,4,3,8,5,9,7,6]
(ejabberd@localhost)5> mnesia:activity(transaction,fun mnesia:select/2,md, [{{md,'$1','_'},[{'<','$1',10}],['$1']}]).
{atomic,[1,2,4,3,8,5,9,7,6]}
%%but when query with select/4,returned [6], why?
(ejabberd@localhost)7> {atomic,{R1,C1}}=mnesia:activity(transaction,fun mnesia:select/4,md,[{{md,'$1','_'},[{'<','$1',10}],['$1']}],5,read).
{atomic,{[6],
{mnesia_select,md,
{tid,10535470,<0.460.0>},
ejabberd@localhost,disc_only_copies,
{dets_cont,select,5,
<<0,0,0,29,18,52,86,120,0,0,0,21,131,104,3,...>>,
{141720,148792,<<>>},
md,<0.130.0>,<<>>},
[],undefined,undefined,
[{{md,'$1','_'},[{'<','$1',10}],['$1']}]}}}
%% and then use mnesia:select/1 with continuation "C1",got wrong_transaction
(ejabberd@localhost)8> mnesia:activity(transaction,fun mnesia:select/1,C1).
{aborted,wrong_transaction}如何使用mnesia:select/4和mnesia:select/1进行分页查询？
回答 1
Stack Overflow用户
回答已采纳
发布于 2013-08-17 13:06:16
以下是我的解决方案:使用async_dirty而不是transaction
{Record,Cont}=mnesia:activity(async_dirty, fun mnesia:select/4,[md,[{Match_head,[Guard],[Result]}],Limit,read])然后阅读下一个限制记录数：
mnesia:activity(async_dirty, fun mnesia:select/1,[Cont])完整代码：
-record(md,{id,name}).
batch_delete(Id,Limit) ->
Match_head = #md{id='$1',name='$2'},
Guard = {'<','$1',Id},
Result = '$_',
{Record,Cont} = mnesia:activity(async_dirty, fun mnesia:select/4,[md,[{Match_head,[Guard],[Result]}],Limit,read]),
delete_next({Record,Cont}).
delete_next('$end_of_table') ->
over;
delete_next({Record,Cont}) ->
delete(Record),
delete_next(mnesia:activity(async_dirty, fun mnesia:select/1,[Cont])).
delete(Records) ->
io:format("delete(~p)~n",[Records]),
F = fun() ->
[ mnesia:delete_object(O) || O <- Records]
end,
mnesia:transaction(F).页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/18205689
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