db file parallel read for prefetching?

hi, all,
   Does anyone has document/note talking about the index prefetching?
I saw a few doc talking about oracle 9i introduced this feature, but
no doc really talked,  how it works, and how to enable/hint the 
prefetch for the optimizer, and how to verify it is working.
   I have two database with identical hardware/schema, and some SQL
with identical plan (and nearly same data volume), SQL in one database
always run at 1/2 time of the other database.
   I checked the 10046 trace and found the database faster has pretty
much "db file parallel read" and the other instance is all "db file
sequential read". I guess it is related to the prefetch, but do not
have much detail about it.

If someone has some experience it, can you share?

--
Regards
Zhu Chao
www.cnoug.org
--
http://www.freelists.org/webpage/oracle-l


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