Details
Description
With the introduction of the index cache to the catalog cache, getting the routing information fetches the placement information from the collection cache and then the index information from the index cache. However, there can be no global indexes on an unsharded collection, so we can forgo fetching the index information from the cache if the placement information we found is unsharded.
This is important because it means that for unsharded collections, we will do one less trip to the config server for a newly accessed collection (we are pushing the first refresh of the index information for when the collection becomes sharded).