This is just a short note about one of the potential side-effects of the new Auto Degree Of Parallelism (DOP) feature introduced in 11.2.
If you happen to have Parallel DML enabled in your session along with Auto DOP (and here I refer to the PARALLEL_DEGREE_POLICY = AUTO setting, not LIMITED) then it might take you by surprise that INSERT statements that are neither decorated with a parallel hint nor use any parallel enabled objects can be turned into direct-path inserts.
I mentioned the day before Open World I put a Virtual RAC on Oracle Linux 6.1 article live. Although the procedure was complete, some of the screen shots were from an old article as I didn’t have time to redo them before my flight. I’ve just run through the procedure again and taken new screen shots. As a result, I’ve allowed the article to display on the front page of the website, which is why you will see it listed as a new article there.
This kinda rounds out the whole Oracle on 6.1 stuff as there has been a single instance installation guide out for ages and more recently the Cloud Control installation, which references it.
Remember, it’s still not certified yet, but it’s coming.
I will be talking about “Administering Parallel Execution in RAC” with demos on Sunday morning 9AM-10AM (session id 28060). This is part of IOUG RAC SIG presentation series. You would enjoy the content and demos I have prepared.
I know, it is too early, but hoping to see you there!
BTW, if you have attended my RAC Advanced Troubleshooting class series, please don’t hesitate to introduce yourself when we meet.
Session ID: 28060
Session Title: IOUG: Administering Parallel Execution in Oracle RAC
Venue / Room: Moscone West- 2005
Date and Time: 10/2/11, 9:00 – 10:00
Update: I just completed this session in IOUG. Thank you for coming, if you were in the room.
You can download the pdf file from
PX execution in RAC
Preface (with apologies to Kevin Closson)
This blog post is too long
In the previous part of this series I've already demonstrated that the logical I/O optimization of the Table Prefetching feature depends on the order of the row sources - and 11g takes this approach a big step further.
It is very interesting that 11g does not require any particular feature like Table Prefetching or Nested Loop Join Batching (another new feature introduced in 11g) to take advantage of the Logical I/O optimization - it seems to be available even with the most basic form of a Nested Loop join.
I've already outlined in one of my previous posts that getting a reasonable cardinality estimate for multi-column joins can be tricky, in particular when dealing with correlated column values in the join columns.
Since Oracle 10g several "Multi-Column Join Cardinality" sanity checks have been introduced that prevent a multi-column join from producing too low join cardinalities - this is controlled via the "_optimizer_join_sel_sanity_check" internal parameter that defaults to true from 10g on.
This is just a short heads-up note regarding a bug that obviously has been introduced with 18.104.22.168: If you happen to have a public synonym for a table that is called differently than the original object then dynamic sampling will not work in 22.214.171.124.
The reason is that the generated query used for the dynamic sampling does not resolve the synonym name properly - it resolves the object owner but uses the synonym name instead of the actual table name. The same issue happens by the way when using a private synonym, however the query is then still valid and works even when using the synonym name.
The bug can only be reproduced in 126.96.36.199, in all previous versions including 188.8.131.52 the synonym resolution seems to work as expected for the dynamic sampling query, so it seems to be a problem introduced in that patch set.
This is just a short note prompted by a recent thread on the OTN forums. In recent versions Oracle changes the costs of a full table scan (FTS or index fast full scan / IFFS) quite dramatically if the "flashback query" clause gets used.
It looks like that it simply uses the number of blocks of the segment as I/O cost for the FTS operation, quite similar to setting the "db_file_multiblock_read_count" ("dbfmbrc"), or from 10g on more precisely the "_db_file_optimizer_read_count", to 1 (but be aware of the MBRC setting of WORKLOAD System Statistics, see comments below) for the cost estimate of the segment in question.
This can lead to some silly plans depending on the available other access paths as can be seen from the thread mentioned.
The Cost Based Optimizer (CBO) supports since at least Oracle 9i the automatic generation of additional predicates based on transitive closure.
In principle this means:
If a = b and b = c then the CBO can infer a = c
As so often with these optimizations the purpose of these automatically generated additional predicates is to allow the optimizer finding potentially more efficient access paths, like an index usage or earlier filtering reducing the amount of data to process.
In the first part of this post I've explained some of the details and underlying reasons of bug 6918210. The most important part of the bug is that it can only be hit if many row migrations happen during a single transaction. However, having excessive row migrations is usually a sign of poor design, so this point probably can't be stressed enough:
If you don't have excessive row migrations the bug can not become significant
Of course, there might be cases where you think you actually have a sound design but due to lack of information about the internal workings it might not be obvious that excessive row migrations could be caused by certain activities.
This was meant to be published shortly after my latest quiz night post as an explanatory follow up, but unfortunately I only managed to complete this note by now.
There is a more or less famous bug in ASSM (see bug 6918210 in MOS as well as Greg Rahn's and Jonathan Lewis' post) in versions below 11.2 that so far has been classified as only showing up in case of a combination of larger block sizes (greater the current default of 8K) and excessive row migrations. With such a combination it was reproducible that an UPDATE of the same data pattern residing in an ASSM tablespace caused significantly more work than doing the same in a MSSM tablespace, because apparently ASSM had problems finding suitable blocks to store the migrated rows.