On Thu, Jan 23, 2014 at 8:51 AM, Graeme Defty <[email protected]> wrote: > HI Evgeny, > > Yes, Mnesia does not have a 'count' function, so I did a 'quick and dirty' > to implement the boss_db functionality. It was not something I needed so I > did not worry about it too much at the time. (Actually I thought I was the > only one using Mnesia!) > > Short of enhancing Mnesia itself I am not sure how to improve this. :-( > > g > >
For mnesia, I wonder if http://www.erlang.org/doc/apps/mnesia/Mnesia_chap4.html#id74480 pattern matching can help here? First to limit, then to select where pattern does not match the first id's in the first limit. Just a thought anyway. > > > > On 23 January 2014 20:40, Evgeny M <[email protected]> wrote: >> >> 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/ >> >> -- >> You received this message because you are subscribed to the Google Groups >> "ChicagoBoss" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected]. >> Visit this group at http://groups.google.com/group/chicagoboss. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/chicagoboss/abd60772-8956-49b6-ba66-273e057fb286%40googlegroups.com. >> For more options, visit https://groups.google.com/groups/opt_out. > > > -- > You received this message because you are subscribed to the Google Groups > "ChicagoBoss" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > Visit this group at http://groups.google.com/group/chicagoboss. > To view this discussion on the web visit > https://groups.google.com/d/msgid/chicagoboss/CAKF5fiCtr9iQqtCoJaMheCCHcvetOCKwgFQmsLt-wLJV6EnM_w%40mail.gmail.com. > > For more options, visit https://groups.google.com/groups/opt_out. -- You received this message because you are subscribed to the Google Groups "ChicagoBoss" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. Visit this group at http://groups.google.com/group/chicagoboss. To view this discussion on the web visit https://groups.google.com/d/msgid/chicagoboss/CA%2BH5g6oBmrkN3yFkK8CiC4fANNLJ%3DGEYPCMWseOi5kXm1tJq9g%40mail.gmail.com. For more options, visit https://groups.google.com/groups/opt_out.
