Topic: Counting the number of distinct values (NDV) for a table column has important applications in the database domain, ranging from query optimization to optimizing reports for large data warehouses. However the legacy SQL method of using SELECT COUNT (DISTINCT <COL>) can be very slow. This is a well known problem and Oracle 12.1.0.2 provides a new function APPROX_COUNT_DISTINCT implemented with a new-generation algorithm to address this issue by providing fast and scalable cardinality estimates.

Maybe you are wondering why a java entry in a databases blog? The answer is simple, at CERN, the Infrastucture and Middleware Services section (aka IMS) is part of the  Database Services , and, among many other things, we take care of the Java application servers for our users.

In my previous blog post (http://db-blog.web.cern.ch/blog/szymon-skorupinski/2014-05-migrating-oracle-database-12c-what-do-auditing) I provided you with number of reasons why unified auditing looks very promising and should be seriously considered while migrating to 12c. Nonetheless, I was not talking at all about performance – which also seems to be greatly improved.

Indexes… what indexes?

Indexes are one of the key Oracle features in terms of improving query performance and one might think it should be clear to everybody when and how to use them. Not at all... Working as a DBA for several years I was surprised to notice that so many developers do not know what indexes are, how they work and how to use them efficiently. I thought it would be good to put this knowledge down in a series of blog posts… and here we are!  Let’s start with…

If you plan to introduce changes in your environment and want to estimate their impact, Real Application Testing feature seems to be one of the best options. As we needed to check the influence of changes planned in our databases, I've started to look for good candidates to capture the workloads. I wanted to capture only workloads associated with small number of schemas, but from several databases, to be able to properly simulate as much types of production workloads existing in our databases as possible.

If you consider migrating to Oracle Database 12c, no matter if you think of development, testing or production environments, one of many decisions you need to take is to define what to do with the auditing.