Joins over histograms

Here's an awesome paper from Alberto Dell'Era and Wolfgang Breitling looking at "the formula used by the Cost Based Optimizer to estimate the cardinality of a [single-column] equijoin, when both the columns referenced in the join predicate have histograms collected."

Awesome because of the thought given to the presentation, and the care taken to test the results. Alberto has followed the lead taken by Jonathan Lewis's CBO book to carefully deconstruct and reverse engineer the algorithms used - with one or two surprising results.

Well worth a read...