V$px_session show no slave sessions but query is executed in parallel

Hi All, 

The interesting thing I encountered these days is although the v$px_session didn't show any slaves, it seemed the query was executed in parallel.

The evidence is: I ran the same sql with and without 'alter session disable parallel query' alternately. The reponse times are stable and quite different.

I have tested another method to check slave processes, that is:
1. To get the sql_id from v$session where sid = my_session_id.
2. To check how many sessions in the database, by select * from gv$session where sql_id = sql_id_from_step1

Both of the ways show no slave processes for my query.