Penalty for reduce?

[Brian Candler]

I'm just wondering, how much is the performance penalty for adding reduce to
a view? In other words, is there any reason *not* to include a reduce
function like

function(ks, vs, co) {
  if (co) {
    return sum(vs);
  } 
  else {
    return vs.length;
  }
}

in every map view I create? (Apart from the fact I have to remember to
supply reduce=false!) Even if all the keys are distinct, this gives the
ability to count all the keys, and to count keys in arbitary ranges.

Might it be useful to include such a reduce in the _all_docs pseudo-view?

Regards,

Brian.