QLC power

I've been doing some more work on my query DSL (described in an earlier post) for an internal project at my work.

I had a number of 2GB sized Mnesia disk_only tables that I wanted to traverse in a QLC query as one single table. Writing a QLC handler solved that very nicely:

tables() ->
    Tables = all_tables(),
    TF = fun() -> next(Tables) end,
    InfoFun = fun(keypos) -> 2;
                 (_)      -> undefined
              end,
    qlc:table(TF, [{info_fun, InfoFun}]).


next([T|_] = Tables) ->
    next(Tables, mnesia:dirty_first(T));
next([]) ->
    [].

next([_|Ts], '$end_of_table') ->
    next(Ts);
next([T|_] = Tables, Key) ->
    [hd(mnesia:dirty_read(T, Key)) |
     fun() -> next(Tables, mnesia:dirty_next(T,Key)) end];
next([], '$end_of_table') ->
    [].

So now I can make QLC queries like:

[T#tab.name || T <- my:tables(), T#tab.age > 42]

Hurray for QLC!

--Tobbe

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