an useful article http://dbakevlar.blogspot.com/2010/02/lead-developer-and-i-have-been-working.html
I love parallel. I am thrilled with the performance gains I can attain when hashing large tables when accessing multiple partitions. I do not love parallel when someone decides to put a group by or order by into the statement. If any sorting has to occur on the statement, I've been seeing some odd behavior in about 80% of the executions. It appears that when sorting, it's not Oracle sorting the data all at once, but sorting one parallel slave's data set at a time. This creates an escalation in temp space usage that can become insanely difficult for Oracle to process efficiently and performance degrades immensely.
Carefully test any parallel processing when sorting is involved. You may not receive performance benefits as expected.
Partitioning -- 3b : More Indexes on Partitioned Table - In the previous blog post, I had demonstrated a Global (Non-Partitioned) Index and a Local (Partitioned) Index. A Global Index itself may be partitioned. ...
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