Home -> Mailing Lists -> Oracle-L -> RE: Crazy dynamic sampling?

RE: Crazy dynamic sampling?

From: Dominic Brooks <dombrooks_at_hotmail.com>
Date: Tue, 17 Apr 2018 09:48:02 +0000
Message-ID: <DB6P190MB0501DE219F8EE910E93FFF7AA1B70_at_DB6P190MB0501.EURP190.PROD.OUTLOOK.COM> 








Yes. It’s querying various subpartitions of a big table.
Which it didn’t do before the hash partitioning of the PK.



Hoping the optimizer trace will reveal something.





From: Stefan Knecht <knecht.stefan_at_gmail.com>
Sent: Tuesday, April 17, 2018 10:37:00 AM
To: Dominic Brooks


Cc: oracle-l-freelists


Subject: Re: Crazy dynamic sampling?



Did you run a 10046 on it? That would show what it's doing and where the time spent parsing is going.



On Tue, Apr 17, 2018 at 4:14 PM, Dominic Brooks <dombrooks_at_hotmail.com<mailto:dombrooks_at_hotmail.com>> wrote:
It’s not the subsequent mutexes that I’m bothered about.
More about why the session at the head of the queue is hard parsing for minutes or hours.





From: Stefan Knecht <knecht.stefan_at_gmail.com<mailto:knecht.stefan_at_gmail.com>>
Sent: Tuesday, April 17, 2018 10:09:35 AM



To: Dominic Brooks; oracle-l-freelists


Subject: Re: Crazy dynamic sampling?



Not seen that on 11.2.0.4.



What mutex and location exactly are the sessions waiting on? You can hit x$mutex_sleep from the p2 in v$session for the cursor: pin S wait on X event. Some more details on how to do thatare here:  WAITEVENT: "cursor: pin S wait on X" Reference Note (Doc ID 1298015.1)



Then I'd search MoS for bugs related to that mutex location.





On Tue, Apr 17, 2018 at 3:55 PM, Dominic Brooks <dombrooks_at_hotmail.com<mailto:dombrooks_at_hotmail.com>> wrote:
Good question – sorry for missing that out.



11.2.0.4





From: Stefan Knecht <knecht.stefan_at_gmail.com<mailto:knecht.stefan_at_gmail.com>>
Sent: Tuesday, April 17, 2018 9:54:57 AM
To: Dominic Brooks; oracle-l-freelists


Subject: Re: Crazy dynamic sampling?



Are you on 12.1.0.2 ?





On Tue, Apr 17, 2018 at 3:49 PM, Dominic Brooks <dombrooks_at_hotmail.com<mailto:dombrooks_at_hotmail.com>> wrote:



In addition to invisible indexes not being so invisible, I hash partitioned some global PK indexes to reduce insert hot block contention.



These two things – invisible local indexes with some unusable subpartitions and hash partitioned PK indexes - are the only things which changed.





I’ve now got a few handfuls of statements which, in addition to some of them doing table expansion thanks to the not-so-invisible invisible indexes, are doing crazy long hard parsing – like 30 to 40 minutes. And that causes knock-on effects to other sessions in the same workflow for the same sql / objects with “cursor: pin S wait on X” and “library cache lock”





The likely culprit is dynamic sampling. I can see the sessions doing lots of IO against different table partitions as part of the hard parse.



Otherwise the health of the database is no less healthy than it was before and the SQL is the same SQL as it was before.







Whilst I wait for an optimizer trace file and some other dumps/traces, any wild theories as to why just hash partitioning a global PK might cause this?









From: Dominic Brooks <dombrooks_at_hotmail.com<mailto:dombrooks_at_hotmail.com>>
Sent: Tuesday, April 10, 2018 11:09:03 AM
To: oracle-l_at_freelists.org<mailto:oracle-l_at_freelists.org>
Subject: Re: Invisible indexes and table expansion



Probably just covered by bug 16544878.



Sent from my iPhone



> On 10 Apr 2018, at 10:50, Dominic Brooks <dombrooks_at_hotmail.com<mailto:dombrooks_at_hotmail.com>> wrote:

>

> Just working through a problem, gathering evidence and trying to reproduce. And it’s difficult for me to get an optimiser trace file in the environment.


>

> In an 11.2.0.4 environment, I’ve got a whole bunch of queries now using table expansion - VW_TE_2.

>

> Recent change was to add three local indexes, each with a subset of partitions as usable - something which table expansion was designed to help with.

>

> But they are invisible indexes.

>

> So my theory is the invisibility is limited - ie they are visible enough to cause table expansion - but then can’t be used.

>

> Anyone looked into something similar?

>

> Cheers

> Dominic

>

> Sent from my iPhone





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Received on Tue Apr 17 2018 - 11:48:02 CEST












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