diff --git a/datumaro/components/dataset.py b/datumaro/components/dataset.py
index 3be34a4a4398..7072150f2df6 100644
--- a/datumaro/components/dataset.py
+++ b/datumaro/components/dataset.py
@@ -266,7 +266,8 @@ def is_cache_initialized(self) -> bool:
 
     @property
     def _is_unchanged_wrapper(self) -> bool:
-        return self._source is not None and self._storage.is_empty()
+        return self._source is not None and self._storage.is_empty() and \
+            not self._transforms
 
     def init_cache(self):
         if not self.is_cache_initialized():
@@ -513,16 +514,17 @@ def get_subset(self, name):
         return self._merged().get_subset(name)
 
     def subsets(self):
-        subsets = {}
-        if not self.is_cache_initialized():
-            subsets.update(self._source.subsets())
-        subsets.update(self._storage.subsets())
-        return subsets
+        # TODO: check if this can be optimized in case of transforms
+        # and other cases
+        return self._merged().subsets()
 
     def transform(self, method: Transform, *args, **kwargs):
         # Flush accumulated changes
-        source = self._merged()
-        self._storage = DatasetItemStorage()
+        if not self._storage.is_empty():
+            source = self._merged()
+            self._storage = DatasetItemStorage()
+        else:
+            source = self._source
 
         if not self._transforms:
             # The stack of transforms only needs a single source
diff --git a/datumaro/components/extractor.py b/datumaro/components/extractor.py
index 26506b30021c..17220d04afc4 100644
--- a/datumaro/components/extractor.py
+++ b/datumaro/components/extractor.py
@@ -686,6 +686,9 @@ def get_subset(self, name):
         if self._subsets is None:
             self._init_cache()
         if name in self._subsets:
+            if len(self._subsets) == 1:
+                return self
+
             return self.select(lambda item: item.subset == name)
         else:
             raise Exception("Unknown subset '%s', available subsets: %s" % \
diff --git a/datumaro/plugins/coco_format/importer.py b/datumaro/plugins/coco_format/importer.py
index ca76dc55db38..44a4c7362190 100644
--- a/datumaro/plugins/coco_format/importer.py
+++ b/datumaro/plugins/coco_format/importer.py
@@ -73,8 +73,12 @@ def __call__(self, path, **extra_params):
 
     @classmethod
     def find_sources(cls, path):
-        if path.endswith('.json') and osp.isfile(path):
-            subset_paths = [path]
+        if osp.isfile(path):
+            if len(cls._TASKS) == 1:
+                return {'': { next(iter(cls._TASKS)): path }}
+
+            if path.endswith('.json'):
+                subset_paths = [path]
         else:
             subset_paths = glob(osp.join(path, '**', '*_*.json'),
                 recursive=True)
diff --git a/tests/test_dataset.py b/tests/test_dataset.py
index d141d00d8840..e594a87f260e 100644
--- a/tests/test_dataset.py
+++ b/tests/test_dataset.py
@@ -867,6 +867,120 @@ def transform_item(self, item):
 
         self.assertEqual(iter_called, 1)
 
+    @mark_requirement(Requirements.DATUM_GENERAL_REQ)
+    def test_can_get_len_after_local_transforms(self):
+        iter_called = 0
+        class TestExtractor(Extractor):
+            def __iter__(self):
+                nonlocal iter_called
+                iter_called += 1
+                yield from [
+                    DatasetItem(1),
+                    DatasetItem(2),
+                    DatasetItem(3),
+                    DatasetItem(4),
+                ]
+        dataset = Dataset.from_extractors(TestExtractor())
+
+        class TestTransform(ItemTransform):
+            def transform_item(self, item):
+                return self.wrap_item(item, id=int(item.id) + 1)
+
+        dataset.transform(TestTransform)
+        dataset.transform(TestTransform)
+
+        self.assertEqual(iter_called, 0)
+
+        self.assertEqual(4, len(dataset))
+
+        self.assertEqual(iter_called, 1)
+
+    @mark_requirement(Requirements.DATUM_GENERAL_REQ)
+    def test_can_get_len_after_nonlocal_transforms(self):
+        iter_called = 0
+        class TestExtractor(Extractor):
+            def __iter__(self):
+                nonlocal iter_called
+                iter_called += 1
+                yield from [
+                    DatasetItem(1),
+                    DatasetItem(2),
+                    DatasetItem(3),
+                    DatasetItem(4),
+                ]
+        dataset = Dataset.from_extractors(TestExtractor())
+
+        class TestTransform(Transform):
+            def __iter__(self):
+                for item in self._extractor:
+                    yield self.wrap_item(item, id=int(item.id) + 1)
+
+        dataset.transform(TestTransform)
+        dataset.transform(TestTransform)
+
+        self.assertEqual(iter_called, 0)
+
+        self.assertEqual(4, len(dataset))
+
+        self.assertEqual(iter_called, 2)
+
+    @mark_requirement(Requirements.DATUM_GENERAL_REQ)
+    def test_can_get_subsets_after_local_transforms(self):
+        iter_called = 0
+        class TestExtractor(Extractor):
+            def __iter__(self):
+                nonlocal iter_called
+                iter_called += 1
+                yield from [
+                    DatasetItem(1),
+                    DatasetItem(2),
+                    DatasetItem(3),
+                    DatasetItem(4),
+                ]
+        dataset = Dataset.from_extractors(TestExtractor())
+
+        class TestTransform(ItemTransform):
+            def transform_item(self, item):
+                return self.wrap_item(item, id=int(item.id) + 1, subset='a')
+
+        dataset.transform(TestTransform)
+        dataset.transform(TestTransform)
+
+        self.assertEqual(iter_called, 0)
+
+        self.assertEqual({'a'}, set(dataset.subsets()))
+
+        self.assertEqual(iter_called, 1)
+
+    @mark_requirement(Requirements.DATUM_GENERAL_REQ)
+    def test_can_get_subsets_after_nonlocal_transforms(self):
+        iter_called = 0
+        class TestExtractor(Extractor):
+            def __iter__(self):
+                nonlocal iter_called
+                iter_called += 1
+                yield from [
+                    DatasetItem(1),
+                    DatasetItem(2),
+                    DatasetItem(3),
+                    DatasetItem(4),
+                ]
+        dataset = Dataset.from_extractors(TestExtractor())
+
+        class TestTransform(Transform):
+            def __iter__(self):
+                for item in self._extractor:
+                    yield self.wrap_item(item, id=int(item.id) + 1, subset='a')
+
+        dataset.transform(TestTransform)
+        dataset.transform(TestTransform)
+
+        self.assertEqual(iter_called, 0)
+
+        self.assertEqual({'a'}, set(dataset.subsets()))
+
+        self.assertEqual(iter_called, 2)
+
     @mark_requirement(Requirements.DATUM_GENERAL_REQ)
     def test_raises_when_repeated_items_in_source(self):
         dataset = Dataset.from_iterable([DatasetItem(0), DatasetItem(0)])