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Expert Oracle SQL

Troubleshooting Oracle Performance, 2nd Edition

STATISTICS COLLECTOR Row Source Operation and LOBs

The aim of the STATISTICS COLLECTOR row source operation, which is used in adaptive plans, is to buffer all data produced by its child operation until it is known whether the inflection point is crossed. It goes without saying that buffering requires memory and, therefore, Oracle Database has to limit the amount of memory that can be allocated for that purpose. As a result, in some situations no adaptive plans can be used because according (more...)

The Importance of the In-Memory DUPLICATE Clause for a RAC System

With the INMEMORY clause you can specify 4 sub-clauses:

  • The MEMCOMPRESS clause specifies whether and how compression is used
  • The PRIORITY clause specifies the priority (“order”) in which the segments are loaded when the IMCS is populated
  • The DISTRIBUTE clause specifies how data is distributed across RAC instances
  • The DUPLICATE clause specifies whether and how data is duplicated across RAC instances

The aim of this post is not to describe these attribues in detail. Instead, (more...)

About Oracle’s Code of Ethics, MOS Confidentiality Clause, and Content Stolen from My Website

I strive to keep the number non-technical posts on this blog to the minimum. However, there are days when a non-technical post has to be written. This is one of such days.

Oracle Corporation, concerned about legal and ethical issues, not only provides to its employees a document entitled Code of Ethics and Business Conduct, but also expects that every employee adheres with the standards that the document sets. For the purpose of (more...)

The APPROX_COUNT_DISTINCT Function – A Test Case

The aim of this post is not to explain how the APPROX_COUNT_DISTINCT function works (you find basic information in the documentation and in this post written by Luca Canali), but to show you the results of a test case I run to assess how well it works.

Here’s what I did…

I created a table with several numerical columns (the name of the column shows how many distinct values it contains), loaded 100 million (more...)


Every new Oracle Database patch introduces not only documented features, but also undocumented ones. In this short post I would like to provide information about an enhancement of the RESULT_CACHE hint.

According the documentation, the RESULT_CACHE hint has no parameter. But, as of, at least the SNAPSHOT parameter is supported. Its purpose is to specify how much time (in seconds) after the creation the result cache entry has to (more...)

Troubleshooting Oracle Performance, 2nd Edition – Downloadable Files

This is just a short post to point out that the downloadable files are available here. You can download either a ZIP or a TAR file containing all files, or single files.

Since every file has its own URL, you can also directly executed them in SQL*Plus. For example:

SQL> @

I’m sorry for the delay!

Differences between the First and the Second Edition of Troubleshooting Oracle Performance

Several people asked me how much I changed in the second edition. The short answer is… A LOT.

The main goals set for the revision of the book were the following:

  • Add content about Oracle Database 11g Release 2 and Oracle Database 12c Release 1.
  • Remove content about Oracle9i and Oracle Database 10g Release 1.
  • Add content that was “missing” in the first edition (for example, features like hierarchical profiler, active session history, AWR and (more...)

Troubleshooting Oracle Performance, 2nd Edition

Few days ago the second edition of Troubleshooting Oracle Performance was finally published. Today, I updated the pages on this site that provide information about the book. It goes without saying that the new pages cover the second edition. Hence, from here you can access the description, the table of contents, the forewords written by Jonathan Lewis and Cary Millsap, as well as a page with some errata. Yes, as for the first edition, (more...)

Upcoming Speaking Engagements

After about four months without any speaking engagements, over the next three months I’ll have the opportunity to speak at several events. I’m looking forward to meet you at one of them!

April 3, I’ll be in Baden-Dättwil to speak about “ASH Analytics” at an event organized by SOUG. Since this is a new presentation covering some stuff that I wrote for the second edition of TOP, I’m looking forward to see how it works.