[erlang-questions] QLC Cursor confusion

Ignas Vyšniauskas <>
Wed Feb 22 15:38:34 CET 2012


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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