Add index to local_group_updates.stream_id (#11231)

This should speed up startup times and generally increase performance of
groups.

synapse/storage/databases/main/group_server.py

@@ -13,15 +13,20 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from typing import Any, Dict, List, Optional, Tuple
+from typing import TYPE_CHECKING, Any, Dict, List, Optional, Tuple
 
 from typing_extensions import TypedDict
 
 from synapse.api.errors import SynapseError
 from synapse.storage._base import SQLBaseStore, db_to_json
+from synapse.storage.database import DatabasePool
+from synapse.storage.types import Connection
 from synapse.types import JsonDict
 from synapse.util import json_encoder
 
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
 # The category ID for the "default" category. We don't store as null in the
 # database to avoid the fun of null != null
 _DEFAULT_CATEGORY_ID = ""
@@ -35,6 +40,16 @@ class _RoomInGroup(TypedDict):
 
 
 class GroupServerWorkerStore(SQLBaseStore):
+    def __init__(self, database: DatabasePool, db_conn: Connection, hs: "HomeServer"):
+        database.updates.register_background_index_update(
+            update_name="local_group_updates_index",
+            index_name="local_group_updates_stream_id_index",
+            table="local_group_updates",
+            columns=("stream_id",),
+            unique=True,
+        )
+        super().__init__(database, db_conn, hs)
+
     async def get_group(self, group_id: str) -> Optional[Dict[str, Any]]:
         return await self.db_pool.simple_select_one(
             table="groups",