I think Mark's comments are probably in the right area.
If you update the table in one session, then other sessions
are going to start creating read-consistent clones. Do you
have a recycle pool ? If not, then the clones will end up in
the Keep pool.

It's possible that if you generate too many clones of blocks
1 to 100 (say) then the "originals" of some other blocks have
to be kicked out of memory to make way for them - and then
get read back later.

You could check with a query like:

select   file#, block#, count(*)
from   v$bh
where   objd = {data object id of table}
group by
     file#, block#
having
    count(*) > 1
/

to see how many copies there are of each block.

Or possibly:
select
ct, count(*)
from
(
select file#, block#, count(*) ct
from v$bh
where objd = {data object id of table}
group by
file#, block#
)
group by ct
/

To get a complete distribution pattern of how heavily
cloned the blocks are.

The queries might be a little brutal on your buffer cache
for several seconds, though.


Regards

Jonathan Lewis
http://jonathanlewis.wordpress.com

Author: Cost Based Oracle: Fundamentals
http://www.jlcomp.demon.co.uk/cbo_book/ind_book.html

The Co-operative Oracle Users' FAQ
http://www.jlcomp.demon.co.uk/faq/ind_faq.html


---- Original Message ----- -

> a.     block cleanouts could cause some blocks to be updated (I think
> that would be a worst case of doubling.)

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