Re: Performance of mnesia:select/2

Without doing measurements, I would guess the second one,
you can combine that with taking a read table_lock first in your
transaction.



On Fri, Feb 26, 2021 at 11:03 AM Vance Shipley <[email protected]> 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
>