Performance of mnesia:select/2

[Dan Gudmundsson]

Interesting, and times do you get for 500 ets lookup on that data?

On Fri, Feb 26, 2021 at 3:12 PM Jacob <jacob01@REDACTED> wrote:

> 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
>
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