Mark Townsend wrote:
> Joel Garry wrote:
>
>> Seemed clear enough to me. ACID only applies to a stable set of data.
>> The "performance tool" is looking at transient data. Any time you
>> start getting down into Oracle internals you will have to deal with
>> this. You can't stop the clay pigeon at your favorite spot and then
>> shoot it.
>
>
> There is definately some heisenberg effects going on here.
>
> I spent some time with the developer that reviewed the bug today, and he
> managed to convince me that it wasn't a bug - an explanation as below.
> But now, after a whisky or two, I note that the bug as logged against
> the tar doesn't quite reflect what was reported in the tar, and the
> explanation he gave me doesn't quit reflect what was actually logged in
> the bug.
>
> So I will continue to follow up. But in a nutshell, here's the
> explanation as to what we thought was reported.
>
> 1) In a RAC environment, any query against a gv$session view will spawn
> one parallel query slave per instance to do the scan. (i.e Session count
> = 1 PQ slave/node)
>
> 2) If you union all two of these queries together without using the WITH
> clause, you will get one parallel query slave per 'branch' of the union
> all, again per instance (i.e Session count = 2 PQ slaves/node)
>
> 3) Adding the WITH clause changes the execution plan so that the second
> (and any subsequent) branch of the query trys to use the intermediate
> results produced by the first, which are materialized to a temp table.
> As such, a query slave for the second branch (the second query in the
> UNION ALL) is now not required, so is not spawned. So with the addition
> of the WITH, session count goes back to 1 PQ slave/node
>
> None of this above is a bug. In fact this is what is supposed to happen.
> Which is why the bug was closed.
>
> Problem is I now have trouble relating this back to what was acutally
> reported in the TAR, and then in the newsgroup, and now my head hurts
> and I think the cat just disappeared :-(
Macallan ... at least 18 years old will cure it. You have my word on
that.
--
Daniel A. Morgan
University of Washington
damorgan_at_x.washington.edu
(replace 'x' with 'u' to respond)
Received on Thu Mar 10 2005 - 22:35:21 CST
