dirty_select a limited number of records

[Roberto Ostinelli]

Here's another try, this one too is very slow:

QH = qlc:q([E || E <- mnesia:table(my_table), E#my_table.pid == Pid]),
mnesia:async_dirty(fun() ->
    QC = qlc:cursor(QH),
    R = qlc:next_answers(QC, N),
    qlc:delete_cursor(QC),
    R
end).

Any ideas?



On Tue, Nov 26, 2019 at 6:33 PM Roberto Ostinelli <ostinelli@REDACTED>
wrote:

> All,
> I've got a mnesia table that I create with:
>
> mnesia:create_table(my_table, [
>     {type, set},
>     {attributes, record_info(fields, my_table)},
>     {index, [#my_table.pid]},
>     {storage_properties, [{ets, [{read_concurrency, true},
> {write_concurrency, true}]}]}
> ]).
>
> If I want to get all records for the secondary index, I do:
>
> mnesia:dirty_index_read(my_table, Pid, #my_table.pid).
>
> This operation if very fast. Now, sometimes I just need the *first*
> matching record and if there are many entries with the secondary index, the
> operation becomes a bottleneck. So I'm trying:
>
> S = fun() ->
>     MatchHead = #my_table{pid = Pid, _ = '_'},
>     Guards = [],
>     Result = '$_',
>     mnesia:select(my_table, [{MatchHead, Guards, [Result]}], 1, read)
> end,
> case mnesia:activity(sync_dirty, S) of
>     {[Entry], _} -> Entry;
>     _ -> undefined
> end.
>
> This works, however this operation does not seem to be using the secondary
> index as it becomes extremely slow. I also tried:
>
> S = fun() ->
>     MatchHead = #my_table{pid = '$1', _ = '_'},
>     Guard = {'=:=', '$1', Pid},
>     Result = '$_',
>     mnesia:select(my_table, [{MatchHead, [Guard], [Result]}], 1, read)
> end,
> case mnesia:activity(sync_dirty, S) of
>     '$end_of_table' -> undefined;
>     {[Entry], _} -> Entry
> end.
>
> And this is also slow. What can I do to dirty_select a limited number of
> records from a mnesia table?
>
> Thank you,
> r.
>
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