RE: Crazy dynamic sampling?

From: Dominic Brooks <dombrooks_at_hotmail.com>
Date: Fri, 20 Apr 2018 10:12:56 +0000
Message-ID: <DB6P190MB0501325A64EB5E7C68D835DFA1B40_at_DB6P190MB0501.EURP190.PROD.OUTLOOK.COM>





Itís been bothering me that I could not reproduce this parsing problem in a UAT database.

So I marked one of the subpartitions on an INVISIBLE index as UNUSABLE. Itís a bit like 12c partial indexes... there are three indexes each built USABLE on a subset of subpartitions.

The original intention was that these indexes Ė A, B, C - each cover all the subpartitions within a partition Ė i.e. index A on all the subpartitions within partition X, index B on all the subpartitions within partition Y and index C on all the subpartitions within partition Z. They have similar leading columns, different trailing columns, because of the nature of the data in the partitions. But with one of them, there wasnít enough TEMP to build one of the subpartitions in prod but the queries in question didnít need that subpartition so we skipped it.

And now Iíve gone back and marked that subpartition as UNUSABLE. I know have that parsing problem in UAT. Which is a good thing.

So I suspect bug 16544878 is at least as relevant as 19631234.

Interesting.

From: Dominic Brooks<mailto:dombrooks_at_hotmail.com> Sent: 18 April 2018 10:58
Subject: RE: Crazy dynamic sampling?

After looking at some oradebug short_stacks output and reviewing that optimizer trace and the dynamic sampling queries, initial suggestion from Oracle is that this is bug 19631234 Ė suboptimal execution plans for dynamic sampling queries - a description which certainly fits the bill at least... affects >= 11.2.0.4 and < 12.2.0.1

From: Andy Sayer <andysayer_at_gmail.com> Sent: Tuesday, April 17, 2018 3:10:11 PM To: Dominic Brooks
Cc: oracle-l-freelists
Subject: Re: Crazy dynamic sampling?

It seems my memory was a little fuzzy. I double checked my notes. At the time, we were running 11.2.0.3. The hints in the dynamic sampling query were pretty much the same. Opt_dyn_sample (yes, thatís a comment not a hint) and no_parallel_index.

It had decided to do a Union all block per partition using extended partition syntax (from table_name partition (partition_name)), with each block having a full(ďSCHEMAĒ.ĒTABLE_NAMEĒ) hint.

This resulted in index full scans of the global indexes per partition (with a filter on tbl$or$idx$part$num..rowid). Oh hang on... there were also full partition table scans for some partitions. I hadnít noticed that before, it was last year so I canít see why this was the case, itís possible there were some unusable partitions of a local index, but Iíd be completely guessing.

Is that what you saw?

We gathered statistics as they were obviously missing and needed gathering anyway. It solved the issue and we havenít seen it since (although we soon after upgraded to 12.1.0.2).

Regards,
Andrew

On Tue, 17 Apr 2018 at 13:28, Dominic Brooks <dombrooks_at_hotmail.com<mailto:dombrooks_at_hotmail.com>> wrote: Thanks... interesting... no, no global statistics on the index. Yes, can get them gathered.

From: oracle-l-bounce_at_freelists.org<mailto:oracle-l-bounce_at_freelists.org> <oracle-l-bounce_at_freelists.org<mailto:oracle-l-bounce_at_freelists.org>> on behalf of Andy Sayer <andysayer_at_gmail.com<mailto:andysayer_at_gmail.com>> Sent: Tuesday, April 17, 2018 11:55:16 AM To: oracle-l-freelists

Subject: Re: Crazy dynamic sampling?
Iíve experienced this before, the global statistics of the indexes in question were missing.

It is certainly buggy looking behaviour for the dynamic sampling to do full index scans, if you trace the execution you will see (or rather I saw in my case) that the dynamic sampling queries were hinting for full index scans. I didnít get as far as looking on MOS but I suspect there will be something there.

Are you able to gather/set the statistics at a global level?

Anecdotally, it happened to me while inserting into a table over a DB link, one of the recursive queries during the parse was a select * from table_at_remote, which was pretty strange. Donít think the issue is limited to this though.

Hope that helps.
Andrew





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Received on Fri Apr 20 2018 - 12:12:56 CEST

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