Don't use hashes for primary keys because they are verbose and they cause a huge amount of cache churn ( http://news.ycombinator.com/item?id=122869 http://news.

Don't use hashes for primary keys because they are verbose and they cause a huge amount of cache churn ( http://news.ycombinator.com/item?id=122869 http://news.ycombinator.com/item?id=122869 ). Use a composite key instead.