Statistical Assignment Policy
The statistical policy does not map a URI to a forest. Instead, each forest keeps track of how many documents it has and broadcasts that information to the other forests through heartbeats. The rebalancer then moves documents from the forests that have the most documents to the forests that have the least number of documents. When a new forest is added, the statistical policy moves the least number of documents to get to a balanced state. All forests don't have to have the exact same amount of documents for a database to be considered “balanced.”
For example, as shown in the figure below, a new forest, ForestD, is added to the database that already has three forests: ForestA, ForestB, and ForestC, each contains 400 documents. Each of the existing forests move 100 documents to the new forest, ForestD.
Note
The number of documents in above example is used for the purposes of illustrating the behavior of the rebalancer when the statistical policy is set. In practice, it is inefficient to move such a small number of documents between forests. Typically, you will not see any significant rebalancing of documents between forests until the number of documents in the database exceeds 100,000.
If your database is balanced (the document count on each forest is roughly the same), setting the assignment policy to statistical will not trigger major data movement and any new inserts from then on will be automatically balanced across the forests.