For each row in a partition, a sliding window of data may be defined. The
window determines a sequence of rows that is used to perform calculations for
the current row. Window sizes can be specified as numbers of rows or can be
determined by intervals in a domain. Either end of a window or both ends can
move, depending on the definition of the window.
Each computation involving an analytic function is based on a current row.
This row serves as reference for the ends of the window.