make the correlation subquery O(1) instead of O(number of snapshots)

Author: tayloraswift

Sources/UnidocQueries/Queries/WideQuery.swift

@@ -102,6 +102,8 @@ extension WideQuery:VolumeLookupQuery
                     $0[let: symbol] = Output.Principal[.master] / Volume.Vertex[.symbol]
                     $0[let: hash] = Output.Principal[.master] / Volume.Vertex[.hash]
 
+                    //  ``namesOfLatest`` is always non-nil, so we don’t need to worry about
+                    //  degenerate index behavior.
                     $0[let: min] = Output.Principal[.namesOfLatest] / Volume.Names[.planes_min]
                     $0[let: max] = Output.Principal[.namesOfLatest] / Volume.Names[.planes_max]
                 }

Sources/UnidocQueries/Volumes/VolumeLookupQuery.swift

@@ -94,6 +94,13 @@ extension VolumeLookupQuery
                 $0[.replaceWith] = .init
                 {
                     $0[Self.names] = Mongo.Pipeline.ROOT
+
+                    //  ``Volume.Names`` is complex but not that large, and duplicating this
+                    //  makes the rest of the query a lot simpler.
+                    if  let names:Mongo.KeyPath = Self.namesOfLatest
+                    {
+                        $0[names] = Mongo.Pipeline.ROOT
+                    }
                 }
             }