Add subject filters in schema relation delete to force use of the index

This should allow for much faster queries under the transaction. We
observed some customers reporting timeouts when writting a new
schema and dropping a relation because it ended up in full table
scans.

internal/services/shared/schema.go

@@ -319,10 +319,46 @@ func sanityCheckNamespaceChanges(
 	for _, delta := range diff.Deltas() {
 		switch delta.Type {
 		case nsdiff.RemovedRelation:
+			// NOTE: We add the subject filters here to ensure the reverse relationship index is used
+			// by the datastores. As there is no index that has {namespace, relation} directly, but there
+			// *is* an index that has {subject_namespace, subject_relation, namespace, relation}, we can
+			// force the datastore to use the reverse index by adding the subject filters.
+			var previousRelation *core.Relation
+			for _, relation := range existing.Relation {
+				if relation.Name == delta.RelationName {
+					previousRelation = relation
+					break
+				}
+			}
+
+			if previousRelation == nil {
+				return nil, spiceerrors.MustBugf("relation `%s` not found in existing namespace definition", delta.RelationName)
+			}
+
+			subjectSelectors := make([]datastore.SubjectsSelector, 0, len(previousRelation.TypeInformation.AllowedDirectRelations))
+			for _, allowedType := range previousRelation.TypeInformation.AllowedDirectRelations {
+				if allowedType.GetRelation() == datastore.Ellipsis {
+					subjectSelectors = append(subjectSelectors, datastore.SubjectsSelector{
+						OptionalSubjectType: allowedType.Namespace,
+						RelationFilter: datastore.SubjectRelationFilter{
+							IncludeEllipsisRelation: true,
+						},
+					})
+				} else {
+					subjectSelectors = append(subjectSelectors, datastore.SubjectsSelector{
+						OptionalSubjectType: allowedType.Namespace,
+						RelationFilter: datastore.SubjectRelationFilter{
+							NonEllipsisRelation: allowedType.GetRelation(),
+						},
+					})
+				}
+			}
+
 			qy, qyErr := rwt.QueryRelationships(ctx, datastore.RelationshipsFilter{
-				OptionalResourceType:     nsdef.Name,
-				OptionalResourceRelation: delta.RelationName,
-			})
+				OptionalResourceType:      nsdef.Name,
+				OptionalResourceRelation:  delta.RelationName,
+				OptionalSubjectsSelectors: subjectSelectors,
+			}, options.WithLimit(options.LimitOne))
 
 			err = errorIfTupleIteratorReturnsTuples(
 				ctx,