How to find the default trace file for the session

Use the v$diag_info view, which "describes the state of Automatic Diagnostic Repository (ADR) functionality"

SELECT value
FROM   v$diag_info
WHERE  name = 'Default Trace File';

Example output:

VALUE
/u01/oracle/diag/rdbms/proddb01/proddb01/trace/proddb01_ora_45082.trc

A real easy way to start a trace of your current session is

exec dbms_monitor.session_trace_enable;

Then execute your statements

When you're done, stop tracing with

exec dbms_monitor.session_trace_disable;