Pagination with mnesia could be quite tricky.
mnesia adapter contains the following function:
count(Conn, Type, Conditions) ->
length(find(Conn, Type, Conditions, all, 0, id, ascending)).
So every time you do Count on mnesia boss_db will load full query result in
memory, with all data of all fields.
And as you do boss_db:find(Model, Conditions, ArgList) again, you will do
the same query and load the same data again.
You may not notice this on developers box when there's only so much of
data, but on real server you will have huge hit on performance and memory.
Of course there's no such problem on postgres or mysql, but count(*)..where
... is quite heavy operation even in RDBMS world
среда, 22 января 2014 г., 19:17:20 UTC+4 пользователь David Welton написал:
>
> Would it be worth including something like this directly in boss_db ?
>
> paginate(Model, Conditions, Args) ->
> Page = proplists:get_value(page, Args, 1),
> PageSize = proplists:get_value(page_size, Args, ?DEFAULT_PAGE_SIZE),
> ArgList = proplists:delete(page_size, proplists:delete(page, Args)) ++
> [{offset, PageSize * (Page - 1)}, {limit, PageSize}],
> Total = boss_db:count(Model, Conditions),
> TotalPages = (Total div PageSize) + (case Total rem PageSize of
> 0 -> 0;
> _ -> 1
> end),
> {Page, TotalPages, boss_db:find(Model, Conditions, ArgList)}.
>
> --
> David N. Welton
>
> http://www.welton.it/davidw/
>
> http://www.dedasys.com/
>
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