I have received the following from Mark J. Bobak an Oracle DBA from ProQuest Information and Learning

"Hi Greg,

If you look at V$SYSSTAT, you can gain some insight to the puzzle:
select name from v$sysstat where name like '%consistent%get%';

Will return (in my 9i database):
consistent gets
consistent gets - examination
no work - consistent read gets
cleanouts only - consistent read gets
rollbacks only - consistent read gets
cleanouts and rollbacks - consistent read gets

This may help you sort out the confusion. I'm not sure I can
explain all of the above, but, let's start w/ the easy one:

'no work - consistent read gets'. This represents the number of times Oracle was doing a consistent read (select) and when it got the buffer, it was consistent w/ the start SCN of the select statement and had been cleaned out, so it didn't need to rollback any changes or cleanout.

'cleanouts only - consistent read gets' would be the number of buffers that Oracle got that needed delayed block cleanout, but not rollback.

'rollbacks only - consistent read gets' is number of buffers that Oracle got that needed rollback but not cleanout.

'cleanouts and rollbacks - consistent read gets' is number of buffers that needed both cleanout and rollback.

I'm not sure what the remaining ones are, and I just tried to get the numbers to add up, and I couldn't, so, there's probably more going on behind the scenes.

But, looking at these numbers, you can get a feel for how much extra work Oracle is doing to support read consistency.

-Mark "

Thanks Mark.

Has anyone else got any additional information.

Thanks.