Fix a bug in the /event_reports Admin API which meant that the total count could be larger than the number of results you can actually query for.

changelog.d/13525.bugfix

@@ -0,0 +1 @@
+Fix a bug in the `/event_reports` Admin API which meant that the total count could be larger than the number of results you can actually query for.

synapse/storage/databases/main/room.py

@@ -2001,9 +2001,15 @@ def _get_event_reports_paginate_txn(
 
             where_clause = "WHERE " + " AND ".join(filters) if len(filters) > 0 else ""
 
+            # We join on room_stats_state despite not using any columns from it
+            # because the join can influence the number of rows returned;
+            # e.g. a room that doesn't have state, maybe because it was deleted.
+            # The query returning the total count should be consistent with
+            # the query returning the results.
             sql = """
                 SELECT COUNT(*) as total_event_reports
                 FROM event_reports AS er
+                JOIN room_stats_state ON room_stats_state.room_id = er.room_id
                 {}
                 """.format(
                 where_clause