19c: Upgrade Performance issue

Hi Everyone!!

Today we were performing 19c upgrade of one of the production database. Interesting issue that we have encountered. Our source system was Oracle 12.1.0.2 and target database version was Oracle 19.15.0.0

After the upgrade all validations completed during the maintenance window, Everyone was happy with the way things went. we have closed our bridge call around 3AMish.  Morning 7ish started getting call and chat pings about the system hang issues reported from several areas.

I have logged into the system to check the OS system usage, uptime command was showing no load at all. No errors on alert log either. Started looking into OEM graphs. Wo!! seeing the wait events. 

Seen wait events on "resmgr:pq queued". Started researching the wait event on Oracle Support portal.

Bug 31620829 - 'resmgr:pq queued' when there are available pq slaves for parallel_force_local queries (Doc ID 31620829.8)

Seems we were hitting the bug 31620829. Really don't have enough time to work on Patch without going through RFC process.  Luckily had workaround to implement.  Bug resolution is with 19.17.x.x

SQL> ALTER SYSTEM SET _parallel_load_publish_threshold=1000 SCOPE=BOTH;

That resolved the issue. Hope this helps someone.