RE: Fun with the CARDINALITY and DYNAMIC_SAMPLING hint
- From: <Christopher.Taylor2@xxxxxxxxxxxx>
- To: <jonathan@xxxxxxxxxxxxxxxxxx>, <oracle-l@xxxxxxxxxxxxx>
- Date: Fri, 14 Sep 2012 08:26:32 -0500
Version 10.2.0.4
That makes good sense - the dynamic sampling would be redundant if you're
telling the optimizer what to use for Cardinality. Hadn't considered that.
Thanks again
Chris
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On
Behalf Of Jonathan Lewis
Sent: Friday, September 14, 2012 8:21 AM
To: oracle-l@xxxxxxxxxxxxx
Subject: Re: Fun with the CARDINALITY and DYNAMIC_SAMPLING hint
Which version are you on.
I've just done a couple of simple checks on 11.2.0.3 and there doesn't seem to
be any unreasonable conflict.
Of course, if you specifiy table-level dynamic sampling and specify the single
table cardinality for the same table then you have to expect one of the hints
to "lose" - and in this case the cardinality hint overrides anything the
dynamic sampling may have done.
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com/all_postings
Author: Oracle Core (Apress 2011)
http://www.apress.com/9781430239543
----- Original Message -----
From: <Christopher.Taylor2@xxxxxxxxxxxx>
To: <oracle-l@xxxxxxxxxxxxx>
Sent: Thursday, September 13, 2012 9:33 PM
Subject: Fun with the CARDINALITY and DYNAMIC_SAMPLING hint
| (Thanks to Jonathan Lewis, Greg Rahn (and others) I've been learning a
lot about tweaking the execution paths for a particular problematic SQL.
| A special thank you to you guys for all the blog posts (a tremendous
source of knowledge) that you write at Oracle Scratchpad and
StructuredData!)
| I was playing around with the hints mentioned in the subject for a
particular problematic query we're working on and using dynamic_sampling I
could reduce the performance from 25M lios to 5M lios (which I think is
still probably too high for this complex DSS query).
|
| However, when I added the CARDINALITY hint, it seems to immediately
disallow the dynamic sampling hint (or something)?
|
| Anyone know if these 2 are compatible? It doesn't seem so.
|
| Also, as a "fun" learning experiment, I played around with cardinality
hints that were completely unrealistic such as 9999999999 (9 billion) -
interesting explain plans doing things like that :) Performance not so
great, but was still interesting to see how you can move tables around in
the xplan based on what you know of them.
|
| Chris
|
|
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