Thank you. And udump is in C:\oracle\admin\mydb ?
I ask this because it has not been created by default.
Should I create it manually before running my query ?
The trace file may not appear until you start doing a little work.
In your version of Oracle you will eventually see two files, one with the suffix .trc - the trace file - and one with the suffix .trm which is a file used by Oracle for their own purposes.
Also in your version of Oracle, the fully qualified name of the trace file will appear in v$process as column tracefile. You could write a high precision query joining v$session for your session to v$process to find exactly the right row, but I usually just query v$process where tracefile like '%ora%' to get the right location then do " ls -ltr *ora* " to pick up the latest list of files.