built-in функции:
github.com/basho/riak_kv/blob/master/priv/mapred_builtins.js
github.com/basho/riak_kv/blob/master/src/riak_kv_mapreduce.erl
Из
Basho Map-Reduce doc:
Finally, if you want your reduce function to be evaluated only once, after all inputs are received, use this argument instead:
{«reduce»:
{...language, etc. as usual…
«arg»:{«reduce_phase_only_1»:true}}}
Similarly, in Erlang:
{reduce, FunSpec, [reduce_phase_only_1], Keep}