diff --git a/swh/core/api/classes.py b/swh/core/api/classes.py
--- a/swh/core/api/classes.py
+++ b/swh/core/api/classes.py
@@ -4,6 +4,7 @@
 # See top-level LICENSE file for more information
 
 from dataclasses import dataclass, field
+import itertools
 from typing import Callable, Generic, Iterable, List, Optional, TypeVar
 
 TResult = TypeVar("TResult")
@@ -18,6 +19,16 @@
     next_page_token: Optional[TToken] = field(default=None)
 
 
+def _stream_results(f, *args, page_token, **kwargs):
+    """Helper for stream_results() and stream_results_optional()"""
+    while True:
+        page_result = f(*args, page_token=page_token, **kwargs)
+        yield from page_result.results
+        page_token = page_result.next_page_token
+        if page_token is None:
+            break
+
+
 def stream_results(
     f: Callable[..., PagedResult[TResult, TToken]], *args, **kwargs
 ) -> Iterable[TResult]:
@@ -26,10 +37,25 @@
     """
     if "page_token" in kwargs:
         raise TypeError('stream_results has no argument "page_token".')
-    page_token = None
-    while True:
-        page_result = f(*args, page_token=page_token, **kwargs)
-        yield from page_result.results
-        page_token = page_result.next_page_token
-        if page_token is None:
-            break
+    yield from _stream_results(f, *args, page_token=None, **kwargs)
+
+
+def stream_results_optional(
+    f: Callable[..., Optional[PagedResult[TResult, TToken]]], *args, **kwargs
+) -> Optional[Iterable[TResult]]:
+    """Like stream_results(), but for functions ``f`` that return an Optional.
+
+    """
+    if "page_token" in kwargs:
+        raise TypeError('stream_results_optional has no argument "page_token".')
+    res = f(*args, page_token=None, **kwargs)
+    if res is None:
+        return None
+    else:
+        if res.next_page_token is None:
+            return iter(res.results)
+        else:
+            return itertools.chain(
+                res.results,
+                _stream_results(f, *args, page_token=res.next_page_token, **kwargs),
+            )
diff --git a/swh/core/api/tests/test_classes.py b/swh/core/api/tests/test_classes.py
--- a/swh/core/api/tests/test_classes.py
+++ b/swh/core/api/tests/test_classes.py
@@ -3,10 +3,12 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
-from typing import TypeVar
+from typing import Optional, TypeVar
+
+import pytest
 
 from swh.core.api.classes import PagedResult as CorePagedResult
-from swh.core.api.classes import stream_results
+from swh.core.api.classes import stream_results, stream_results_optional
 
 T = TypeVar("T")
 TestPagedResult = CorePagedResult[T, bytes]
@@ -21,7 +23,27 @@
     assert list(actual_data) == []
 
 
-def test_stream_results_no_pagination():
+def test_stream_results_optional():
+    def paged_results(page_token) -> Optional[TestPagedResult]:
+        return None
+
+    # Should be None, not an empty iterator!
+    actual_data = stream_results_optional(paged_results)
+    assert actual_data is None
+
+
+@pytest.mark.parametrize("stream_results", [stream_results, stream_results_optional])
+def test_stream_results_kwarg(stream_results):
+    def paged_results(page_token):
+        assert False, "should not be called"
+
+    with pytest.raises(TypeError):
+        actual_data = stream_results(paged_results, page_token=42)
+        list(actual_data)
+
+
+@pytest.mark.parametrize("stream_results", [stream_results, stream_results_optional])
+def test_stream_results_no_pagination(stream_results):
     input_data = [
         {"url": "something"},
         {"url": "something2"},
@@ -35,7 +57,8 @@
     assert list(actual_data) == input_data
 
 
-def test_stream_results_pagination():
+@pytest.mark.parametrize("stream_results", [stream_results, stream_results_optional])
+def test_stream_results_pagination(stream_results):
     input_data = [
         {"url": "something"},
         {"url": "something2"},
diff --git a/swh/core/db/__init__.py b/swh/core/db/__init__.py
--- a/swh/core/db/__init__.py
+++ b/swh/core/db/__init__.py
@@ -211,7 +211,7 @@
         tblname: str,
         columns: Iterable[str],
         cur: Optional[psycopg2.extensions.cursor] = None,
-        item_cb: Optional[Callable[[Any], Any]] = None,
+        item_cb: Optional[Callable[[Mapping[str, Any]], Any]] = None,
         default_values: Optional[Mapping[str, Any]] = None,
     ) -> None:
         """Run the COPY command to insert the `columns` of each element of `items` into