Hugo Kornelis continues a series on execution plan operators:
The Window Spool operator is one of the four spool operators that SQL Server supports. Like other spool operators, it retains a copy of data it receives and can then return those rows as often as needed. The specific functionality of the Window Spool operator allows it to replay rows within a window, as defined in a ROWS or RANGE specification of an OVER clause.
Read on to see how these work, as well as a few differences from their spool brethren.