diff --git a/elasticgraph-graphql/lib/elastic_graph/graphql/schema/type.rb b/elasticgraph-graphql/lib/elastic_graph/graphql/schema/type.rb
index d35e73e88..f502dc850 100644
--- a/elasticgraph-graphql/lib/elastic_graph/graphql/schema/type.rb
+++ b/elasticgraph-graphql/lib/elastic_graph/graphql/schema/type.rb
@@ -61,12 +61,12 @@ def name
         # List of index definitions that should be searched for this type.
         def search_index_definitions
           @search_index_definitions ||=
-            if indexed_aggregation?
-              # For an indexed aggregation, we just delegate to its source type. This works better than
-              # dumping index definitions in the runtime metadata of the indexed aggregation type itself
-              # because of abstract (interface/union) types. The source document type handles that (since
-              # there is a supertype/subtype relationship on the document types) but that relationship
-              # does not exist on the indexed aggregation.
+            if (st = source_type)
+              # When a type has a source type (a prime example being indexed aggregations), we delegate
+              # to the source type. This works better than dumping index definitions in the runtime metadata
+              # of the derived type itself because of abstract (interface/union) types. The source document
+              # type handles that (since there is a supertype/subtype relationship on the document types)
+              # but that relationship does not exist on derived types.
               #
               # For example, assume we have these indexed document types:
               # - type Person {}
@@ -76,7 +76,7 @@ def search_index_definitions
               # We can go from `Inventor` to its subtypes to find the search indexes. However, `InventorAggregation`
               # is NOT a union of `PersonAggregation` and `CompanyAggregation`, so we can't do the same thing on the
               # indexed aggregation types. Delegating to the source type solves this case.
-              @schema.type_named(@object_runtime_metadata.source_type).search_index_definitions
+              st.search_index_definitions
             else
               @index_definitions.union(subtypes.flat_map(&:search_index_definitions))
             end
@@ -117,13 +117,37 @@ def unwrap_fully
           end
         end
 
-        # Returns the subtypes of this type, if it has any. This is like `#possible_types` provided by the
+        # Returns all concrete subtypes, at any depth. This is like `#possible_types` provided by the
         # GraphQL gem, but that includes a type itself when you ask for the possible types of a non-abstract type.
         def subtypes
           @subtypes ||= @schema
             .graphql_schema
             .possible_types(graphql_type, visibility_profile: :boot)
-            .map { |t| @schema.type_from(t) } - [self]
+            .map { |t| @schema.type_from(t) }
+            .reject { |t| t == self }
+            .to_set
+        end
+
+        # For derived types (e.g. indexed aggregations), returns the underlying source document type.
+        # Returns `nil` for non-derived types.
+        def source_type
+          return @source_type if defined?(@source_type)
+          @source_type = @object_runtime_metadata&.source_type&.then { |st| @schema.type_named(st) }
+        end
+
+        # Returns the set of concrete (non-abstract) indexed document types that share any of this
+        # type's search indexes but are not subtypes of this type. Used to determine whether a
+        # `__typename` filter is needed when querying an abstract type.
+        #
+        # Abstract types are excluded because documents in the datastore are always associated
+        # with a concrete `__typename`. When filtering by `__typename`, only concrete types are
+        # relevant.
+        def concrete_non_subtypes_in_shared_index
+          @concrete_non_subtypes_in_shared_index ||=
+            search_index_definitions
+              .flat_map { |index_def| @schema.document_types_stored_in(index_def.name).to_a }
+              .reject { |t| t == self || subtypes.include?(t) || t.abstract? }
+              .to_set
         end
 
         def field_named(field_name)
diff --git a/elasticgraph-graphql/sig/elastic_graph/graphql/schema/type.rbs b/elasticgraph-graphql/sig/elastic_graph/graphql/schema/type.rbs
index a3d840c2f..1436d393d 100644
--- a/elasticgraph-graphql/sig/elastic_graph/graphql/schema/type.rbs
+++ b/elasticgraph-graphql/sig/elastic_graph/graphql/schema/type.rbs
@@ -9,6 +9,9 @@ module ElasticGraph
         attr_reader graphql_type: ::GraphQL::Schema::_Type
         attr_reader grouping_missing_value_placeholder: ::String? | ::Numeric?
         def search_index_definitions: () -> ::Array[DatastoreCore::_IndexDefinition]
+        def source_type: () -> Type?
+        def subtypes: () -> ::Set[Type]
+        def concrete_non_subtypes_in_shared_index: () -> ::Set[Type]
         def unwrap_fully: () -> Type
         def field_named: (::String) -> Field
         def fields_by_name_in_index: () -> ::Hash[::String, ::Array[Field]]
diff --git a/elasticgraph-graphql/spec/unit/elastic_graph/graphql/schema/type_spec.rb b/elasticgraph-graphql/spec/unit/elastic_graph/graphql/schema/type_spec.rb
index 872826847..4a1209326 100644
--- a/elasticgraph-graphql/spec/unit/elastic_graph/graphql/schema/type_spec.rb
+++ b/elasticgraph-graphql/spec/unit/elastic_graph/graphql/schema/type_spec.rb
@@ -604,14 +604,14 @@ def type_for(field_name)
             end
           end
 
-          it "returns [] for object types" do
+          it "returns an empty set for object types" do
             type = schema.type_named("Size")
-            expect(type.subtypes).to eq []
+            expect(type.subtypes).to be_empty
           end
 
-          it "returns [] for scalar types" do
+          it "returns an empty set for scalar types" do
             type = schema.type_named("Int")
-            expect(type.subtypes).to eq []
+            expect(type.subtypes).to be_empty
           end
 
           it "returns the subtypes of a union" do
@@ -758,6 +758,104 @@ def search_index_definitions_from(type_name: "TheType")
           end
         end
 
+        describe "#source_type" do
+          attr_reader :schema
+
+          before(:context) do
+            @schema = define_schema(clients_by_name: {}) do |s|
+              s.enum_type "Status" do |t|
+                t.value "ACTIVE"
+                t.value "INACTIVE"
+              end
+
+              s.object_type "Thing" do |t|
+                t.field "id", "ID!"
+                t.field "status", "Status"
+                t.index "things"
+              end
+            end
+          end
+
+          it "returns the underlying document type for an indexed aggregation type" do
+            expect(schema.type_named("ThingAggregation").source_type).to be schema.type_named("Thing")
+          end
+
+          it "returns nil for a non-derived object type" do
+            expect(schema.type_named("Thing").source_type).to be_nil
+          end
+
+          it "returns nil for a scalar type" do
+            expect(schema.type_named("Int").source_type).to be_nil
+          end
+
+          it "returns nil for an enum type" do
+            expect(schema.type_named("Status").source_type).to be_nil
+          end
+        end
+
+        describe "#concrete_non_subtypes_in_shared_index" do
+          attr_reader :schema
+
+          before(:context) do
+            @schema = define_schema(clients_by_name: {}) do |s|
+              # A root interface with an index shared by all sub-hierarchies.
+              s.interface_type "Channel" do |t|
+                t.field "id", "ID!"
+                t.index "channels"
+              end
+
+              # Sibling types — NOT under Store, but share the Channel index.
+              s.object_type "Wholesaler" do |t|
+                t.implements "Channel"
+                t.field "id", "ID!"
+              end
+
+              s.object_type "Distributor" do |t|
+                t.implements "Channel"
+                t.field "id", "ID!"
+              end
+
+              # A sub-interface and its concrete subtypes.
+              s.interface_type "Store" do |t|
+                t.implements "Channel"
+                t.field "id", "ID!"
+              end
+
+              s.object_type "OnlineStore" do |t|
+                t.implements "Store"
+                t.field "id", "ID!"
+              end
+
+              # PhysicalStore overrides to use a dedicated index.
+              s.object_type "PhysicalStore" do |t|
+                t.implements "Store"
+                t.field "id", "ID!"
+                t.index "physical_stores"
+              end
+            end
+          end
+
+          it "excludes the type itself and its subtypes, returning only concrete sibling types in the shared index" do
+            expect(schema.type_named("Store").concrete_non_subtypes_in_shared_index).to contain_exactly(
+              schema.type_named("Wholesaler"),
+              schema.type_named("Distributor")
+            )
+          end
+
+          it "excludes the type itself even when the type is a concrete type in a shared index" do
+            # Wholesaler is a concrete type stored in the "channels" index, so it appears in
+            # document_types_stored_in("channels"). Without the `t == self` guard it would include itself.
+            expect(schema.type_named("Wholesaler").concrete_non_subtypes_in_shared_index).to contain_exactly(
+              schema.type_named("Distributor"),
+              schema.type_named("OnlineStore")
+            )
+          end
+
+          it "returns an empty set when all types sharing its indexes are subtypes" do
+            expect(schema.type_named("Channel").concrete_non_subtypes_in_shared_index).to be_empty
+          end
+        end
+
         describe "#hidden_from_queries?" do
           it "returns `false` on a type that has no backing indexed types" do
             schema = define_schema do |s|