Comments on: DB_BLOCK_SIZE and DB_FILE_MULTIBLOCK_READ_COUNT 2 – What is Wrong with this Quote?

By: Charles Hooper

Charles Hooper — Mon, 06 Dec 2010 17:59:22 +0000

The page also states the 40% and 7% rules for index/table scans: remote-dba.net/t_tuning_sql_optimizers.htm

By: Charles Hooper

Charles Hooper — Mon, 06 Dec 2010 11:44:13 +0000

As I stated earlier, it is interesting to compare the text in the “Oracle High-Performance SQL Tuning” book, published by Oracle Press (McGraw-Hill) and written by the same author as the “Oracle Tuning the Definitive Reference Second Edition”, with the text in the “Oracle Tuning the Definitive Reference Second Edition” book. In the Oracle Press book the author states as fact that 40% and 7% rules, while in the Rampant book the author claims that “Oracle says” and then lists the same 40% and 7% rules. The author may be refuting his earlier advice, if the advice did not originate from the days of Oracle 5. [On a related note, I once incorrectly paraphrased one of Tom Kyte's books (http://books.google.com/books?id=8_WChqD3nc4C&pg=PA256 ) in a Usenet thread, where I mentioned that his book states that an index access path should not be used when more than 20% of a table will be accessed. I feel fortunate that he saw the Usenet post and responded.]

Once again, the author states that the ALL_ROWS OPTIMIZER_MODE favors, no not favors – is the most common cause of, unnecessary full table scans. Are all full table scans evil? No, and index access paths are not necessarily better than an unnecessary full table scan.

If the OPTIMIZER_MODE being set to ALL_ROWS is not the most common cause of unnecessary full table scans, and missing indexes are not to blame, why isn’t Oracle using my index (http://hoopercharles.wordpress.com/2010/05/25/true-or-false-why-isnt-my-index-getting-used/ )? How should this portion of the book be fixed?

By: Jonathan Lewis

Jonathan Lewis — Sat, 04 Dec 2010 09:10:15 +0000

I tried to find a refernce on My Oracle Support (the documentation graveyard formerly known as Metalink) to see if I could find the reference when I saw the 40% – but I couldn’t it. I think it came from the days of Oracle 5.

By: Charles Hooper

Charles Hooper — Sat, 04 Dec 2010 00:51:06 +0000

This one might be a little more challenging.

To help you get started, notice how the quoted phrase changed from the time that it appeared in the Oracle Press book titled “Oracle 9i High-Performance Tuning with Statspack”. Does the author mean “Oracle Press” when the book states “Oracle says”? If not, where did the 40% figure originate?