calling views in Erlang

[Norman Barker <no...@gmail.com>]

Hi,

I have written some code to query a view within Erlang and it seems
too simple :-) Am I really just looping through a view and returning
{ok, State} if a key matches a particular input KeyId and calling
{stop, State} when I want the iteration to stop?

I appreciate there is an index behind the view which makes things
fast, but the looping seems really simple.  Verification of this
approach would be very much appreciated as I go forward!

       case couch_view:get_map_view(Db, DesignId, ViewName, []) of
           {ok, View, Group} ->
              FoldlFun = fun({{Key, DocId}, Value}, _, {MaxRecords,
StartPos, Counter, DocIdList}=Acc) ->
                 case Key == KeyId of
                   true ->
                      case Counter < StartPos of
                         true ->
                            % not emitting a value yet until we go past StartPos
                            {ok, {MaxRecords, StartPos, Counter + 1,
DocIdList}};
                         _ ->
                            case (Counter - StartPos) < MaxRecords of
                              true ->
                                {ok, {MaxRecords, StartPos, Counter +
1, [DocId] ++ DocIdList}};
                              _ ->
                                % gone past max records
                                {stop, Acc}
                            end
                       end;
                   _ ->
                      {stop, Acc}
                  end
               end,

              FoldAccInit = {MaxRecords, StartPos, 0, []},
              case couch_view:fold(View, {KeyId, nil}, fwd, FoldlFun,
FoldAccInit) of
                 {ok, {_, _, _, DocIdList}} ->
                   io:fwrite("LIST ~p ViewName ~p ~n", [DocIdList, ViewName]);
                 _ ->
                   throw({error, {[{code, "NoApplicableCode"},
{reason, "Error parsing view" ++ binary_to_list(ViewName)}]}})
              end;
           {not_found, Reason} ->
               throw({error, {[{code, "NoApplicableCode"}, {reason, Reason}]}})
        end;


thanks,

Norman

Re: calling views in Erlang

[Paul Davis <pa...@gmail.com>]

On Sat, Nov 7, 2009 at 10:57 PM, Paul Davis <pa...@gmail.com> wrote:
> Norman,
>
>> I have written some code to query a view within Erlang and it seems
>> too simple :-)
>
> Welcome to CouchDB. Simple is the implementation corollary of Relax. :)
>
>> Am I really just looping through a view and returning
>> {ok, State} if a key matches a particular input KeyId and calling
>> {stop, State} when I want the iteration to stop?
>>
>> I appreciate there is an index behind the view which makes things
>> fast, but the looping seems really simple.  Verification of this
>> approach would be very much appreciated as I go forward!
>
> Even in advanced algorithms indexes are basically just sorted lists at
> the core. Couch's view interface makes that pretty plain. So yes, you
> can just iterate over the views as you see fit. The key to why this is
> quick and delicious is that seeking to the start of iteration (when
> you provide a startkey) is that you only do a log(N) seek to the first
> key.
>
> HTH,
> Paul Davis
>

I should mention, that even with no startkey, its still a log(N) seek,
just that its seeking to a special key that sorts before all keys. The
seeking part is important because its assumed that most view
iterations are over small subsets of the index not starting near the
head of the index on average.

Paul Davis

Re: calling views in Erlang

[Paul Davis <pa...@gmail.com>]

Norman,

> I have written some code to query a view within Erlang and it seems
> too simple :-)

Welcome to CouchDB. Simple is the implementation corollary of Relax. :)

> Am I really just looping through a view and returning
> {ok, State} if a key matches a particular input KeyId and calling
> {stop, State} when I want the iteration to stop?
>
> I appreciate there is an index behind the view which makes things
> fast, but the looping seems really simple.  Verification of this
> approach would be very much appreciated as I go forward!

Even in advanced algorithms indexes are basically just sorted lists at
the core. Couch's view interface makes that pretty plain. So yes, you
can just iterate over the views as you see fit. The key to why this is
quick and delicious is that seeking to the start of iteration (when
you provide a startkey) is that you only do a log(N) seek to the first
key.

HTH,
Paul Davis