Add an index to event_search

- to make the purge API quicker

synapse/storage/events.py

@@ -207,6 +207,17 @@ class EventsStore(SQLBaseStore):
             where_clause="contains_url = true AND outlier = false",
         )
 
+        # an event_id index on event_search is useful for the purge_history
+        # api. Plus it means we get to enforce some integrity with a UNIQUE
+        # clause
+        self.register_background_index_update(
+            "event_search_event_id_idx",
+            index_name="event_search_event_id_idx",
+            table="event_search",
+            columns=["event_id"],
+            unique=True,
+        )
+
         self._event_persist_queue = _EventPeristenceQueue()
 
     def persist_events(self, events_and_contexts, backfilled=False):