2 files changed, 10 insertions, 0 deletions
diff --git a/changelog.d/15531.misc b/changelog.d/15531.misc
new file mode 100644
index 0000000000..6d4da961b5
--- /dev/null
+++ b/changelog.d/15531.misc
@@ -0,0 +1 @@
+Speed up deleting of old rows in `event_push_actions`.
diff --git a/synapse/storage/databases/main/event_push_actions.py b/synapse/storage/databases/main/event_push_actions.py
index ab8f354dc1..2e98a29fef 100644
--- a/synapse/storage/databases/main/event_push_actions.py
+++ b/synapse/storage/databases/main/event_push_actions.py
@@ -1612,6 +1612,15 @@ class EventPushActionsWorkerStore(ReceiptsWorkerStore, StreamWorkerStore, SQLBas
             # deletes.
             batch_size = self._rotate_count
 
+            if isinstance(self.database_engine, PostgresEngine):
+                # Temporarily disable sequential scans in this transaction. We
+                # need to do this as the postgres statistics don't take into
+                # account the `highlight = 0` part when estimating the
+                # distribution of `stream_ordering`. I.e. since we keep old
+                # highlight rows the query planner thinks there are way more old
+                # rows to delete than there actually are.
+                txn.execute("SET LOCAL enable_seqscan=off")
+
             txn.execute(
                 """
                 SELECT stream_ordering FROM event_push_actions