Буквально сегодня в oracle-L прилетело:
авторFYI.. the problem was related to In-Memory...and was fixed with

ALTER SYSTEM SET "_inmemory_dynamic_scans"=DISABLE;

Explanation here : https://docs.oracle.com/en/database/oracle/oracle-database/18/inmem/in-memory-column-store-architecture.html#GUID-4DDD1A53-D634-4201-B46F-626DAE16A447
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2.4.3.2.1 About Lightweight Threads

A lightweight thread is an execution entity that helps to parallelize full table
scans. It is “lightweight” because it does not incur the higher memory
overhead of Oracle processes.

Note:
A lightweight thread used by IM dynamic scans is not the same as a regular
thread in the multithreaded Oracle Database model.

Lightweight threads share the resources of the parent foreground or PQ process,
called the table scan process, that coordinates the scan of a set of IMCUs.
Threads maintain their own independent flow of execution. The database can
parallelize scans by prioritizing threads and executing them asynchronously.

For eligible queries, the process allocates a pool of threads. Resource Manager
automatically determines the number of threads in the pool based on the CPU
count in the database host and the current load on the system. The pool of
threads remains available to the session for subsequent queries unless the idle
time reaches an internal threshold, at which point the database terminates the
threads.
^---(*) the threads terminate themselves

Communication between threads occurs exclusively within a process. For this
reason, contention does not occur at the database instance level.
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We shall file a bug report to avoid the excessive trace file generation that really makes no sense here, it is not an error condition, it is an informational trace file.