fix abnormally slow query that was failing to use partial index

Author: tayloraswift

Sources/UnidocQueries/Activity/Unidoc.PackagesCrawledQuery.swift

@@ -59,6 +59,13 @@ extension Unidoc.PackagesCrawledQuery:Mongo.PipelineQuery
             $0[.foreignField] = Unidoc.PackageMetadata[.repo] / Unidoc.PackageRepo[.created]
             $0[.pipeline] = .init
             {
+                /// This improves query performance enormously, as it gets MongoDB to use the
+                /// partial index. But why?? The `localField` is always non-null!
+                $0[.match] = .init
+                {
+                    $0[Unidoc.PackageMetadata[.repo]] = .init { $0[.exists] = true }
+                }
+
                 $0[.count] = count
             }
             $0[.as] = Date[.repos]

Sources/UnidocQueries/Packages/Unidoc.RealmQuery.swift

@@ -63,8 +63,8 @@ extension Unidoc.RealmQuery:Unidoc.AliasingQuery
             }
         }
 
-        //  This *should* be able to use the partial index even without `$exists` guards,
-        //  as `_id` is always present.
+        //  It’s not clear to me how this is able to use the partial index even without
+        // `$exists` guards, but somehow it does.
         pipeline[.lookup] = .init
         {
             $0[.from] = UnidocDatabase.Packages.name