mr.addReducePhase(new NamedErlangFunction("riak_kv_mapreduce", "reduce_set_union"), false);
mr.addMapPhase(new JSSourceFunction("function(v, keyData) { return [{\"step\": keyData, \"v\": v}]; }"),
true);
return mr.execute();
} catch (ConversionException e) {
throw new IOException(e.getMessage());
} catch (RiakException e) {
throw (IOException) e.getCause();
}