Added state_group_events count example

SQL-queries-for-analyze-Matrix-Synapse-PosrgreSQL-database-stats.md

@@ -15,6 +15,13 @@ SELECT r.name, s.room_id, s.state_events
   ORDER BY ts DESC, state_events DESC
   LIMIT 20;
 ```
+and by state_group_events count _(join removed for performance reasons)_:
+```
+SELECT s.room_id, count(s.room_id) FROM state_groups_state s
+GROUP BY s.room_id        
+ORDER BY count(s.room_id) DESC
+LIMIT 20;
+```
 
 ## Show top 20 larger tables by row count
 ```