Avoid unnecessary pg_listening_channels queries

Author: kitogo

asyncpg/connection.py

@@ -1267,9 +1267,9 @@ def terminate(self):
 
     async def reset(self, *, timeout=None):
         self._check_open()
+        reset_query = self._get_reset_query()
         self._listeners.clear()
         self._log_listeners.clear()
-        reset_query = self._get_reset_query()
 
         if self._protocol.is_in_transaction() or self._top_xact is not None:
             if self._top_xact is None or not self._top_xact._managed:
@@ -1476,7 +1476,7 @@ def _get_reset_query(self):
             _reset_query.append('SELECT pg_advisory_unlock_all();')
         if caps.sql_close_all:
             _reset_query.append('CLOSE ALL;')
-        if caps.notifications and caps.plpgsql:
+        if caps.notifications and caps.plpgsql and len(self._listeners.keys()) != 0:
             _reset_query.append('''
                 DO $$
                 BEGIN