Re: Fun with the CARDINALITY and DYNAMIC_SAMPLING hint
Date: Fri, 14 Sep 2012 14:20:41 +0100
Message-ID: <D240FF5106E84E69BEF904BAA5BD43EE_at_Primary>
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_at_parallon.net> To: <oracle-l_at_freelists.org> 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
|
|
-- http://www.freelists.org/webpage/oracle-lReceived on Fri Sep 14 2012 - 08:20:41 CDT