Change to not require a state_groups.room_id index.

This does mean that we won't clean up orphaned state groups (i.e. state
groups that were persisted but the associated event wasn't).

synapse/storage/data_stores/main/events.py

@@ -1624,7 +1624,10 @@ class EventsStore(
         """Deletes all record of a room
 
         Args:
-            room_id (str):
+            room_id (str)
+
+        Returns:
+            Deferred[List[int]]: The list of state groups to delete.
         """
 
         return self.runInteraction("purge_room", self._purge_room_txn, room_id)
@@ -1714,10 +1717,24 @@ class EventsStore(
         #       index on them. In any case we should be clearing out 'stream' tables
         #       periodically anyway (#5888)
 
+        # Now we fetch all the state groups that should be deleted.
+        txn.execute(
+            """
+                SELECT DISTINCT state_group FROM events
+                INNER JOIN event_to_state_groups USING(event_id)
+                WHERE events.room_id = ?
+            """,
+            (room_id,),
+        )
+
+        state_groups = [row[0] for row in txn]
+
         # TODO: we could probably usefully do a bunch of cache invalidation here
 
         logger.info("[purge] done")
 
+        return state_groups
+
     def purge_unreferenced_state_groups(
         self, room_id: str, state_groups_to_delete: Set[int]
     ) -> defer.Deferred:
@@ -1825,54 +1842,53 @@ class EventsStore(
 
         return {row["state_group"]: row["prev_state_group"] for row in rows}
 
-    def purge_room_state(self, room_id):
+    def purge_room_state(self, room_id, state_groups_to_delete):
         """Deletes all record of a room from state tables
 
         Args:
             room_id (str):
+            state_groups_to_delete (list[int]): State groups to delete
         """
 
         return self.runInteraction(
-            "purge_room_state", self._purge_room_state_txn, room_id
+            "purge_room_state",
+            self._purge_room_state_txn,
+            room_id,
+            state_groups_to_delete,
         )
 
-    def _purge_room_state_txn(self, txn, room_id):
+    def _purge_room_state_txn(self, txn, room_id, state_groups_to_delete):
         # first we have to delete the state groups states
         logger.info("[purge] removing %s from state_groups_state", room_id)
 
-        txn.execute(
-            """
-            DELETE FROM state_groups_state
-            WHERE state_group IN (
-                SELECT state_group FROM state_groups
-                WHERE room_id = ?
-            )
-            """,
-            (room_id,),
+        self._simple_delete_many_txn(
+            txn,
+            table="state_groups_state",
+            column="state_group",
+            iterable=state_groups_to_delete,
+            keyvalues={},
         )
 
         # ... and the state group edges
         logger.info("[purge] removing %s from state_group_edges", room_id)
 
-        txn.execute(
-            """
-            DELETE FROM state_group_edges
-            WHERE state_group IN (
-                SELECT state_group FROM state_groups
-                WHERE room_id = ?
-            )
-            """,
-            (room_id,),
+        self._simple_delete_many_txn(
+            txn,
+            table="state_group_edges",
+            column="state_group",
+            iterable=state_groups_to_delete,
+            keyvalues={},
         )
 
         # ... and the state groups
         logger.info("[purge] removing %s from state_groups", room_id)
 
-        txn.execute(
-            """
-            DELETE FROM state_groups WHERE room_id = ?
-            """,
-            (room_id,),
+        self._simple_delete_many_txn(
+            txn,
+            table="state_groups",
+            column="id",
+            iterable=state_groups_to_delete,
+            keyvalues={},
         )
 
     async def is_event_after(self, event_id1, event_id2):