Re: CBO Question - Is 'monitoring' used to calculate query plan?
From: Martin Berger <martin.a.berger_at_gmail.com>
Date: Thu, 8 Jan 2009 21:34:30 +0100
Message-Id: <50124F9C-086D-4C70-8BFC-CDC4616C9D6B_at_gmail.com>
>
> It still doesn't answer the question however of "Does the CBO use
> the stats seen in all_tab_modifications for query plans?"
>
I have never seen such a behaviour (but I haven't tested it yet)
Date: Thu, 8 Jan 2009 21:34:30 +0100
Message-Id: <50124F9C-086D-4C70-8BFC-CDC4616C9D6B_at_gmail.com>
>
> It still doesn't answer the question however of "Does the CBO use
> the stats seen in all_tab_modifications for query plans?"
>
I have never seen such a behaviour (but I haven't tested it yet)
It might be of any sense to improve the affinity for sampling data about a table if the count of modifications is too high, but - never seen something like this.
maybe you can create a testcase (best with a fresh started instance or at least flushed dictionary cache) and event 10046 trace: in this case you will see a lot of queries from sys, where all the information the kernel requires about an object are selected (and loaded into dictionary cache). If the base-tables for *_tab_modifications are part of these queries, they might be of any interrest for any kind of the kernel (not automatically said the CBO!). Otherwise I would be quite sure, at query time noone cares.
hth
Martin
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