Re: Query ASH for Undo Blocks

We normally check the undo read percentage from Ash history by verifying how many samples are with current_obj# as 0. If it's non zero then should be actual table/index block but not undo. And -1 points to on CPU. But yes it won't say if it's 'consistent read related undo' or 'delayed block cleanout related undo'. For that we need to refer the run time session statistics as Nenad pointed out.

On Mon, 13 Jun 2022, 4:32 pm Noveljic Nenad, <nenad.noveljic_at_vontobel.com> wrote:

> I think yes, but it's only useful in the case when the buffer gets result
> in read wait events. But what if all the undo blocks needed are already
> cached?
>
> You can check if the session statistics for "%undo records applied" is
> increasing:
> https://asktom.oracle.com/pls/apex/f?p=100:11:::::P11_QUESTION_ID:9542989000346736054
>
> Alternatively, you can trace the Oracle C function kcbgtcr with bpftrace.
>
> Best regards,
>
> Nenad
>
>
>
>
> -----Original Message-----
> From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> On
> Behalf Of Lothar Flatz
> Sent: Montag, 13. Juni 2022 12:36
> To: oracle-l_at_freelists.org
> Subject: Query ASH for Undo Blocks
>
> *** E-Mail from outside Vontobel: Do not click on links or open
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>
> Hi,
>
> there is a plan with an Index Range scan that generates way more
> buffer_gets than you would expect.
> Would it be correct to query active session history using CURRENT_FILE#
> and CURRENT_BLOCK# to check against DBA_EXTENTS to find out if some buffer
> gets are coming from undo?
> The query does work, but i am not sure if the result is showing the
> indented answer.
>
> Thanks
>
> Lothar
>
>
>
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