At the recent OUG Ireland conference I had the privilege of participating in a panel discussion on the Oracle Database. During the course of the session the topic of Optimizer histograms came up. As always, a heated discussion ensued among the members of the panel, as we each had very different views on the subject.
Why so many different opinions when it comes to histograms?
The problem arises from the fact that some folks have been burnt by histograms in the past. In Oracle Database 9i and 10g, histograms in combination with bind-peeking lead to some unpredictable performance problems, which is explained in detail in this post on the Optimizer blog. This has resulted in a number of folks becoming histogram shy. In fact, I reckon if you were to put 3 Oracle experts on a panel, you would get at least 5 different opinions on when and how you should gather histograms!
So I thought it would be a good idea to explain some of the common misconceptions that surround histograms and the impact of adopting them.
This is a long post, so you might want to grab a coffee before you get into it!