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From "Adam Kocoloski (JIRA)" <j...@apache.org>
Subject [jira] Commented: (COUCHDB-861) Enhance _sum to handle lists of numbers.
Date Thu, 19 Aug 2010 14:42:16 GMT

    [ https://issues.apache.org/jira/browse/COUCHDB-861?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12900291#action_12900291
] 

Adam Kocoloski commented on COUCHDB-861:
----------------------------------------

I worked on something like this with Ben Anderson.  I agree, it's a useful extension.  Would
you want to generalize it so that the lists of different lengths don't crash the reducer?
 Perhaps

sum_terms(Xs, []) ->
    Xs;
sum_terms([], Ys) ->
    Ys;
sum_terms([X|Xs], [Y|Ys]) when is_number(X), is_number(Y) ->
    [X+Y | sum_terms(Xs,Ys)].

> Enhance _sum to handle lists of numbers.
> ----------------------------------------
>
>                 Key: COUCHDB-861
>                 URL: https://issues.apache.org/jira/browse/COUCHDB-861
>             Project: CouchDB
>          Issue Type: Improvement
>          Components: Database Core
>            Reporter: Robert Newson
>
> _sum can currently only sum a single value but it is often useful to emit an array of
numbers on the value side.
> The code below achieves this, I'm working on a test.
> diff --git a/src/couchdb/couch_query_servers.erl b/src/couchdb/couch_query_servers.erl
> index 92c30f8..e2a748e 100644
> --- a/src/couchdb/couch_query_servers.erl
> +++ b/src/couchdb/couch_query_servers.erl
> @@ -167,8 +167,13 @@ builtin_sum_rows(KVs) ->
>      lists:foldl(fun
>          ([_Key, Value], Acc) when is_number(Value) ->
>              Acc + Value;
> +        ([_Key, Value], Acc) when is_list(Value) ->
> +            case Acc of
> +                0 -> Value;
> +                _ -> lists:zipwith(fun(X, Y) -> X + Y end, Acc, Value)
> +            end;
>          (_Else, _Acc) ->
> -            throw({invalid_value, <<"builtin _sum function requires map values
to be numbers
> +            throw({invalid_value, <<"builtin _sum function requires map values
to be numbers
>      end, 0, KVs).
>  
>  builtin_stats(reduce, [[_,First]|Rest]) when is_number(First) ->

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