The space of the private copy of the temporary table is allocated in the temporary tablespace - so it is
allocated one extent at a time as you need the space,
and extra extents are allocated as each extent becomes
By default, the temporary tablespace is locally managed with uniform extents of 1MB.
The Co-operative Oracle Users' FAQ http://www.jlcomp.demon.co.uk/faq/ind_faq.html
Cost Based Oracle: Fundamentals http://www.jlcomp.demon.co.uk/cbo_book/ind_book.html
Thanks for your reply, and I have a question based on your answer.
How does oracle know certain session will insert how much data into the GTT? Suppose the session first inserts 1 row, and 10 minutes later, inserts another 1,000,000 rows. So at the beginning to operate the GTT, how much space will be allocated? If the space one session occupied is not continuous, then I don't think HWM makes much sense.