[erlang-questions] QLC Cursor confusion

Artem Teslenko <>
Wed Feb 22 15:50:47 CET 2012


Hello,

Try to use ets:table instead of mnesia:table

On Wed, 22 Feb 2012, Ignas Vyšniauskas wrote:

> Hi Erlangers,
> 
> I'm somewhat confused with the correct usage of QLC Cursors. I'd like to
> return the cursor (or a equivalently a reference to it), so that the
> receiver could perform consecutive calls to qlc:next_answers(CursorRef,
> N). But as far as I understand, this is not quite possible. Here is me
> failing to make friends with QLC (sorry for the mangled formatting):
> 
> 40> Q = qlc:q([P || P <- mnesia:table('number.info'),
> P#'number.info'.owner_id == <<"13">>]).
> {qlc_handle,{qlc_lc,#Fun<erl_eval.20.21881191>,
>                     {qlc_opt,false,false,-1,any,[],any,524288,allowed}}}
> 41> qlc:cursor(Q).
> 
> ** exception exit: {aborted,no_transaction}
>      in function  mnesia:abort/1
>      in call from mnesia:pre_qlc/2
>      in call from qlc:call/4
>      in call from qlc:table_handle/3
>      in call from qlc:setup_le/3
>      in call from qlc:setup_quals/6
>      in call from qlc:setup_quals/4
>      in call from qlc:setup_le/3
> 
> Okayy, let's try it in a transaction:
> 
> 42> {atomic, QR} = mnesia:transaction(fun() -> qlc:cursor(Q) end).
> 
> {atomic,{qlc_cursor,{<0.436.0>,<0.434.0>}}}
> 
> Cool, it works, but...
> 
> 43> qlc:next_answers(QR, 1).
> ** exception error: {qlc_cursor_pid_no_longer_exists,<0.436.0>}
>      in function  qlc:next_loop/3
> 
> Hm, but...
> 
> 44> mnesia:transaction(fun() -> qlc:next_answers(QR, 1) end).
> {aborted,{not_cursor_owner,[{qlc,next_answers,
>                                  [{qlc_cursor,{<0.436.0>,<0.434.0>}},1]},
>                             {mnesia_tm,apply_fun,3},
>                             {mnesia_tm,execute_transaction,5},
>                             {erl_eval,do_apply,5},
>                             {shell,exprs,7},
>                             {shell,eval_exprs,7},
>                             {shell,eval_loop,3}]}}
> 
> I'm confused. Does that mean the cursor lives in some process owned by
> mnesia and I can only access it from within a single transaction? What
> am I doing wrong here?
> 
> Thanks,
> Ignas
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