RE: Query tuning exercise: what to look for in a 10053 trace
- From: "Schultz, Charles" <sac@xxxxxxxxxxxxx>
- To: "Allen, Brandon" <Brandon.Allen@xxxxxxxxxxx>, <oracle-l@xxxxxxxxxxxxx>
- Date: Tue, 25 Jul 2006 14:04:47 -0500
That makes sense. Since the long-running query has not yet completed, it
is rather hard to know the "real" number of rows at each step (as
opposed to the estimate). The cardinality reported by autotrace (and the
10053 trace) do not differ by much, but I have to assume that this
little difference is having a huge impact on the ordering. Perhaps the
straw that broke the camel's back.
Thanks for the feedback.
________________________________
From: Allen, Brandon [mailto:Brandon.Allen@xxxxxxxxxxx]
Sent: Tuesday, July 25, 2006 1:34 PM
To: Schultz, Charles; oracle-l@xxxxxxxxxxxxx
Subject: RE: Query tuning exercise: what to look for in a 10053 trace
Yep, read them both - and saw his presentation at Hotsos :-) I'm not
sure on this, but I think the join order will be a result of the estd
cardinalities as well - with a NL join, it will start with the row
source that it expects to return the smallest number of rows, then probe
the other table for matching rows. So, if it underestimates the
cardinality, then it will end up doing a lot more probing than it
expected to do.
________________________________
From: Schultz, Charles [mailto:sac@xxxxxxxxxxxxx]
Sent: Tuesday, July 25, 2006 11:25 AM
To: Allen, Brandon; oracle-l@xxxxxxxxxxxxx
Subject: RE: Query tuning exercise: what to look for in a 10053 trace
I see you have been reading his "tuning by cardinality feedback" paper.
Or perhaps "under the hood..."? =)
I have read them, but it takes a long time for me to absorb the
wonderful insights these guys have.
From an academic standpoint, I am trying to figure out why the CBO is
using a different join order - perhaps this is a vain and useless
endeavor, but I am curious.
Privileged/Confidential Information may be contained in this message or
attachments hereto. Please advise immediately if you or your employer do
not consent to Internet email for messages of this kind. Opinions,
conclusions and other information in this message that do not relate to
the official business of this company shall be understood as neither
given nor endorsed by it.
Other related posts:
