MongoQueryIndex
data class MongoQueryIndex(val context: String, val aggregate: String, val name: String, val keys: List<String>, val collection: String = MongoSnapshotStore.collectionName(context, aggregate), partial: Document = MongoSnapshotStore.notDeleted())
A planned query index of aggregate in context: its name, unique within its collection, and its keys in order, each ascending, over the documents its partial filter keeps: on the snapshot collection, the snapshots that are not deleted; on the log collection (MongoQueryIndexes.planLog), the streams.