Re: Query CBO Cost - Hints

<bdbafh_at_gmail.com> wrote in message
news:1112995246.090664.288030_at_f14g2000cwb.googlegroups.com...
> Costs across parsings don't necessarily mean anything.

Hi Paul

They *might* be incomparable, but normally they are comparable within the same system. I've recently seen a number of posts - and a couple of metalink documents - that backup Jonathan's assertion that the cost is a direct estimate of the elapsed time the query will take (in units of the average single block disk access time). Given either no system stats, or consistent system stats - neither a big leap for 9i, then I cannot see any reason why you cannot compare costs over parsings, in the same system. The same plan may have a different cost because of differences in the stats, but the costs *should* in my view be comparable - the only thing that I can think of that should make a difference is a change in system stats.

If anyone has done any work comparing actual elapsed times to cbo costs, I'd be extremely interested to see it. If not I guess there is some work to be done.

-- 
Niall Litchfield
Oracle DBA
http://www.niall.litchfield.dial.pipex.com