diff --git a/synapse/_scripts/synapse_port_db.py b/synapse/_scripts/synapse_port_db.py
index 642fd41629b1..26834a437e9c 100755
--- a/synapse/_scripts/synapse_port_db.py
+++ b/synapse/_scripts/synapse_port_db.py
@@ -418,12 +418,15 @@ async def handle_table(
             self.progress.update(table, table_size)  # Mark table as done
             return
 
+        # We sweep over rowids in two directions: one forwards (rowids 1, 2, 3, ...)
+        # and another backwards (rowids 0, -1, -2, ...).
         forward_select = (
             "SELECT rowid, * FROM %s WHERE rowid >= ? ORDER BY rowid LIMIT ?" % (table,)
         )
 
         backward_select = (
-            "SELECT rowid, * FROM %s WHERE rowid <= ? ORDER BY rowid LIMIT ?" % (table,)
+            "SELECT rowid, * FROM %s WHERE rowid <= ? ORDER BY rowid DESC LIMIT ?"
+            % (table,)
         )
 
         do_forward = [True]