Don't needlessly batch in add_event_to_cache (#10784)

We've already batched up the events previously, and assume in other
places in the events.py file that we have. Removing this makes it easier
to adjust the batch sizes in one place.

changelog.d/10784.misc

@@ -0,0 +1 @@
+Minor speed ups when joining large rooms over federation.

synapse/storage/databases/main/events.py

@@ -1547,35 +1547,32 @@ def _add_to_cache(self, txn, events_and_contexts):
         to_prefill = []
 
         rows = []
-        N = 200
-        for i in range(0, len(events_and_contexts), N):
-            ev_map = {e[0].event_id: e[0] for e in events_and_contexts[i : i + N]}
-            if not ev_map:
-                break
-
-            sql = (
-                "SELECT "
-                " e.event_id as event_id, "
-                " r.redacts as redacts,"
-                " rej.event_id as rejects "
-                " FROM events as e"
-                " LEFT JOIN rejections as rej USING (event_id)"
-                " LEFT JOIN redactions as r ON e.event_id = r.redacts"
-                " WHERE "
-            )
 
-            clause, args = make_in_list_sql_clause(
-                self.database_engine, "e.event_id", list(ev_map)
-            )
+        ev_map = {e.event_id: e for e, _ in events_and_contexts}
+        if not ev_map:
+            return
 
-            txn.execute(sql + clause, args)
-            rows = self.db_pool.cursor_to_dict(txn)
-            for row in rows:
-                event = ev_map[row["event_id"]]
-                if not row["rejects"] and not row["redacts"]:
-                    to_prefill.append(
-                        _EventCacheEntry(event=event, redacted_event=None)
-                    )
+        sql = (
+            "SELECT "
+            " e.event_id as event_id, "
+            " r.redacts as redacts,"
+            " rej.event_id as rejects "
+            " FROM events as e"
+            " LEFT JOIN rejections as rej USING (event_id)"
+            " LEFT JOIN redactions as r ON e.event_id = r.redacts"
+            " WHERE "
+        )
+
+        clause, args = make_in_list_sql_clause(
+            self.database_engine, "e.event_id", list(ev_map)
+        )
+
+        txn.execute(sql + clause, args)
+        rows = self.db_pool.cursor_to_dict(txn)
+        for row in rows:
+            event = ev_map[row["event_id"]]
+            if not row["rejects"] and not row["redacts"]:
+                to_prefill.append(_EventCacheEntry(event=event, redacted_event=None))
 
         def prefill():
             for cache_entry in to_prefill: