event 10053 – obtaining optimizer computations

to see how the oracle optimizer generates the execution plan  use the event 10053

alter session set max_dump_file_size = unlimited;
alter session set tracefile_identifier='tr10053';
alter session set events='10053 trace name context forever, level 1';

the tracefile will be written to user_dump_dest (10.2) or in <diag_dest>/rdbms/<database>/<instance>/trace