For windowed functions, SQL Server introduces two new operators in the execution plan; Segment and Sequence Project. If you’ve tried looking them up in the documentation, you’ll know that it’s not exactly perfectly obvious how they work. Here’s my stab at clarifying what they actually do.
rank
An introduction to windowed functions
Windowed functions are a powerful feature of T-SQL, allowing you to perform advanced aggregates. They provide a very efficient way of doing this as soon as you just get the hang of the OVER() clause.
sqlsunday.com 