“PGA memory operation” is just an new wait event to expose a prior unmeasured wait in earlier versions of the database. High wait times for that event likely mean the underlying OS/hardware is becoming CPU or memory starved. In customer’s 12.2.0.1 environment, the Available Physical Memory size was very low compared to the blocks size of TABLE ACCESS FULL and MERGE JOIN operation. Therefore the overhead of memory resource operation in OS side spent a lot of CPU time.