Fun with the CARDINALITY and DYNAMIC_SAMPLING hint
- From: <Christopher.Taylor2@xxxxxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Thu, 13 Sep 2012 15:33:15 -0500
(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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