Flink SQL, How can i set parallelism in clause of group by ?

[forideal]

Hello everyone


     I'm a Flink SQL user. Now i have a question.How can i set parallelism in 
clause of group by.
     For example
     SELECT 
            T.user_id,
            D.user_name
     (SELECT 
             user_id,
             MIN(processtime)
     from my_table 
    group by user_id,TUMBLE(processtime, INTERVAL'15'SECOND)) AS T
     LEFTJOIN my_dim_table FOR SYSTEM_TIME ASOF T.proc_time AS D ON T.user_id = 
D.user_id
     
     If the partition of the topic is 3, then,the parallelism of the job is 3.
     In this example,there are three operator
         0ne is Source operator, parallelism is 3
         Two is GroupWindowAggregate operator,parallelism is 3
         Three is LookupJoin operator,parallelism is 3


      I want to change the parallelism of GroupWindowAggregate,but i can't.


Best wishes
forideal