I got a few requests for additional topics where one can read about the SQL Server query optimizer’s search framework (which happens to be called Cascades). I will warn you that most of these are academic papers. Some of these use different terminology than the SQL Server Books Online and therefore may or may not make a lot of sense.
This is a paper I co-wrote a few years ago that talks about how SQL Server optimizer deals with distributed queries. It contains a section that is a general overview of the search framework (at a slightly deeper level of detail than the talk I gave at SQL Bits, for those who attended)
Another paper that some of my colleagues wrote on SQL Update processing in SQL Server:
(I am unsure if this is publicly available or is behind a paywall)
This is another paper talking about the Cascades framework