Friday, March 11, 2022

PX Deq: Signal ACK RSG

PX Deq: Signal ACK RSG wait event does not have much information available even on My Oracle Support portal. I was not able to find much information about this. I noticed this wait event at the top of wait events in my AWR report as can be seen bellow that made me do some investigation. First and the foremost, this wait event is related to parallel execution of SQLs.

AWR report

Further checking revealed that there were top SQLs that seem to be stuck for a very long time as number of executions were also zero.


And when I checked these SQLs, they were the SQLs querying gv$session dynamic performance view.

Point to be noted here is that whenever a query is executed on gv$* views, oracle internally spans parallel query processes to execute the queries. My application module was executing these queries that were fetching data from GV$SESSIN view and apparently module execution got stuck somewhere, and queries were in hung state, thus making this parallel wait event appear at the top.

If you see similar type of situation, try to find out queries involving fetching data form any GV$* views and kill those sessions and re-execute the module/application

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