Shard Keys

Shard a collection on a field with three distinct values, and the cluster can never divide the data into more than three useful parts. We tried this with a collection called bystatus, whose 20,000 documents hold a counter i and a status of pending, paid or shipped. After each value had its own chunk, MongoDB refused to divide further:

db.adminCommand({ split: "store.bystatus", find: { status: "paid" } })
// MongoServerError: Unable to find median in chunk because chunk is indivisible.

That limit stays as the collection grows, and every document with