Why is only the suffix of work_index hashed?

I'm reading through the PDF that Brett Slatkin has published for Google I/O 2010:

"Data pipelines with Google App Engine".开发者_Python百科

In this video (the Fan-in part) Brett says that the work_index has to be a hash, so that 'you distribute the load across the BigTable'... and this is how work_index is created:

work_index = '%s-%d' % (sum_name, knuth_hash(index))

...which I guess creates something like 'mySum-54657651321987'

I do understand the basic idea, but is why only one half of work_index is hashed? Is it important to hash only part of it leaving the suffix out? Would it be wrong to do

md5('%s-%d' % (sum_name, index)) so that the hash would be like '6gw8....hq6'

?

I'm Java guy so I would use md5 to hash, which means I get id like 'mySum' + 32 characters. (Obviously I want my ids/keys to be as short as possible here.) If I could hash the whole string my id would be just 32 chars.

Or would you suggest to use something else to do the hashing with?

Brett Slatkin's own explanation