Q&A: Oracle Performane Tuning

Why using globe temp table (GTT)?
Faster, improve performance.
http://www.dba-oracle.com/t_temporary_tables_sql.htm
http://www.dba-oracle.com/t_sql_rewrite_temporary_tables.htm

What is the difference between 'on commit preserve rows' and 'on commit delete rows' for GTT?
The ON COMMIT PRESERVE ROWS makes this a session based temporary table. rows will stay in this table until a logoff.
The ON COMMIT DELETE ROWS makes this a transaction based temp table. When you commit -- the rows disappear.
http://asktom.oracle.com/pls/apex/f?p=100:11:0::NO::P11_QUESTION_ID:48812348054

What is explain plan and how to read it?
It tells you how oracle processes the query. Read it form inner right to outer left.
http://www.oracle-base.com/articles/8i/ExplainPlanUsage.php
http://www.orafaq.com/node/1420

Why use bulk collect?
Improve performance.
http://www.dba-oracle.com/t_oracle_bulk_collect.htm