Optimizer Costing 1 – What is Wrong with this Quote?

5 12 2010

December 5, 2010 (Modified December 6, 2010)

(Forward to the Next Post in the Series)

I recently reviewed the book “Oracle Tuning the Definitive Reference Second Edition”, and did not provide an in-depth technical review of the entire book.  As I stated in a comment in the earlier blog article, I would like to clarify that my review on the Amazon site is not intended to suggest that people should not buy the book. As the publisher’s website states that the book is written for senior Oracle DBAs, I suggest that senior DBAs, Oracle Certified Masters, and other people who are able to research the book’s contents *should* buy the book and post reviews of the book that highlight specific content of the book indicating whether or not that content is correctly stated. Such feedback will be a benefit to the Oracle community, and could help to improve Oracle Database books that are published in the future. I think that we need to keep a positive outlook in order to see things improve

With that said, what is wrong with the following quote from page 498 of the book (you might need to view the page directly in the book to fully determine the context of the quote)?

“CPU Based Optimizer Costing

The recently added CPU costing feature, controlled by the _optimizer_cost_model = hidden parameter, enhances the CBO’s capabilities by allowing it to estimate the number of machine cycles necessary for an operation…  Generally, CPU costs are not considered significant unless the entire Oracle instance is using excessive CPU resources.

I/O Costing

… The I/O cost is proportional to the number of physical data blocks read by the operation.  However, the CBO has no prior information on the data buffer contents and cannot distinguish between a logical read (in-buffer) and a physical read.  Due to this shortcoming, the CBO cannot know if the data blocks are already in the RAM data buffers.  The best environment for using CPU costing is for all_rows execution plans, where cost is more noteworthy than with first_rows optimization.”

What, if anything, is wrong with the above quote from the book?

The point of blog articles like this one is not to insult authors who have spent thousands of hours carefully constructing an accurate and helpful book, but instead to suggest that readers investigate when something stated does not exactly match what one believes to be true.  It could be that the author “took a long walk down a short pier”, or that the author is revealing accurate information which simply cannot be found through other resources (and may in the process be directly contradicting information sources you have used in the past).  If you do not investigate in such cases, you may lose an important opportunity to learn something that could prove to be extremely valuable.

A Google book search indicates that a slightly modified version of the statement appeared in one other book:

Other pages found during a Google search of the phrase:

  • rampant-books.com/t_oracle_cpu_costing.htm
  • dba-oracle.com/art_builder_cpu_io.htm
  • dba-oracle.com/art_otn_cbo_p6.htm
  • Using Explain Plan from the Oracle documentation library

I am reminded of a quote from page 7 of the book, quite literally the first sentence in the book:

“Oracle tuning is a complex endeavor, and it does not help that Oracle databases are changing constantly.”

Edit: December 6, 2010: Added a couple of more quoted sentences under the heading “CPU Based Optimizer Costing” to reduce the risk of misunderstanding the context of the quote.



2 responses

6 12 2010
Charles Hooper

I might not have included enough text from the book. I tried to help clarify the context of the quote. Note that under the heading “CPU Based Optimizer Costing”, the book states that this feature is controlled by the _OPTIMIZER_COST_MODEL hidden parameter.

7 12 2010
Chris Saxon

I feel that the quotes have some items slightly back-to-front.

As I understand it, the optimizer calculates the CPU and I/O costs for the possible methods of accessing the requested data, then chooses the access path with the lowest cost. Therefore costing is always “significant” as even small changes in cost could cause an execution plan to change, if the original costs were similar.

The all_rows and first_rows hints are instructions to optimize for fetching all the data or just the first (n) rows respectively. Fetching all the data in a set is likely to have a higher cost, as more work is done compared to returning just n rows. Whether this makes the cost more “noteworthy” is debatable. If my above understanding is correct, the cost is always calculated relative to the other access methods for a given environment. Therefore you cannot directly compare the costs for first_rows vs all_rows for a given statement, as these changes to the environment affect how the CBO makes it’s decision and therefore which plan it chooses.

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