Re: Performance of mnesia:select/2

Hi,

assuming that the match spec compiler does clever things with patterns,
I'd use select with the following

   MatchExpression = [ {{'_', K, '_'}, [], ['$_']} || K <- Keys ]

If did some quick measurements with plain ETS, timer:tc and 500 keys out
of 1000000 table entries and got:

   * using pattern, [ordered_set]: 4532605 us
   * using pattern, [set]              :  4645525 us
   * using pattern, [ordered_set, {keypos, 2}]: 3826 us (!!!)
   * using pattern, [set, {keypos, 2}]: 5714 us (!!!)

   * using guards, [ordered_set]: 12542928 us
   * using guards, [set]:               12310452 us
   * using guards, [ordered_set, {keypos, 2}]: 12365477 us
   * using guards, [set, {keypos, 2}]: 12277839 us

I have initialised the DB with [ ets:insert(t, {N, N,
integer_to_list(N)}) || N <- lists:seq(1, 1000000) ].

I don't know though, how this will translate to Mnesia, but I'd give
select with pattern on the primary key a try.

/Jacob


On 2/26/21 11:03 AM, Vance Shipley wrote:
> If I need to lookup a list of keys which is the better approach? Why?
>
> Fselect = fun(Keys) ->
>         MatchHead = {'_', '$1', '$2'},
>         F = fun(Key) ->
>                 {'=:=', '$1', Key}
>         end,
>         MatchConditions = [list_to_tuple(['or' | lists:map(F, Keys)]),
>         MatchBody = ['$_'],
>         MatchFunction = {MatchHead, MatchConditions, MatchBody},
>         MatchExpression = [MatchFunction],
>         mnesia:select(Table, MatchExpression)
> end,
> mnesia:transaction(Fselect, [Keys]).
>
> Fread = fun F([Key | T], Acc) ->
>                 [R] = mnesia:read(Table, Key),
>                 F(T, [R | Acc]);
>         F([], Acc) ->
>                 lists:reverse(Acc)
> end,
> mnesia:transaction(Fread, [Keys. []]).
>
>
> --
>      -Vance