Avoid unnecessary database access.

synapse/handlers/relations.py

@@ -273,13 +273,28 @@ async def _get_threads_for_events(
         # Fetch thread summaries.
         summaries = await self._main_store.get_thread_summaries(event_ids)
 
-        # Only fetch participated for a limited selection based on what had
+        # Only fetch whether the current user has participated in a thread based
+        # on which returned for a limited selection based on what had
         # summaries.
         thread_event_ids = [
             event_id for event_id, summary in summaries.items() if summary
         ]
-        participated = await self._main_store.get_threads_participated(
-            thread_event_ids, user_id
+
+        # Preseed thread participation with whether the requester send the event.
+        participated = {
+            event_id: events_by_id[event_id].sender == user_id
+            for event_id in thread_event_ids
+        }
+        # Check other events against the database.
+        participated.update(
+            await self._main_store.get_threads_participated(
+                [
+                    event_id
+                    for event_id in thread_event_ids
+                    if not participated[event_id]
+                ],
+                user_id,
+            )
         )
 
         # Then subtract off the results for any ignored users.

tests/rest/client/test_relations.py

@@ -1073,7 +1073,7 @@ def assert_thread(bundled_aggregations: JsonDict) -> None:
             return assert_thread
 
         # A user which has sent the root event or replied has participated.
-        self._test_bundled_aggregations(RelationTypes.THREAD, _gen_assert(True), 10)
+        self._test_bundled_aggregations(RelationTypes.THREAD, _gen_assert(True), 9)
         # Note that this re-uses some cached values, so the total number of
         # queries is much smaller.
         self._test_bundled_aggregations(
@@ -1131,7 +1131,7 @@ def assert_thread(bundled_aggregations: JsonDict) -> None:
                 bundled_aggregations["latest_event"].get("unsigned"),
             )
 
-        self._test_bundled_aggregations(RelationTypes.THREAD, assert_thread, 10)
+        self._test_bundled_aggregations(RelationTypes.THREAD, assert_thread, 9)
 
     def test_nested_thread(self) -> None:
         """