Best practice for partial cartesian product

[Reinis Vicups]

Hi,

this is not mahout question directly, but I figured that you guys most 
likely can answer it.

Actually I have two questions:

1. This: {(1,2); (1,3); (2,3)} is not full cartesian product, right? It 
is missing (1,1); (2,2); (3,3); (2,1);.... My question is - how is it 
called? Partial cartesian? Asymetric cartesian?

2. If I try to build the product I described above in reducer, what 
would be the best practice? My current code look like this:

    @Override
    public void reduce(final VarLongWritable key, final 
Iterable<VarLongWritable> values, final Context context)  {

        final VarLongWritable[] valueArray = Iterables.toArray(values, 
VarLongWritable.class);

        for (int i = 0; i < valueArray.length; i++) {
            for (int j = i + 1; j < valueArray.length; j++) {
                context.write(new PairWritable(valueArray[i].get(), 
valueArray[j].get()), customerPreferenceWritable);
            }
        }
    }

I don't feel quite right with this solution since I make a copy of 
values in "valueArray" and believe that it will cost me 
OoutOfMemoryExceptions with larger data sets.

thanks and br
reinis