SQL Server typically operates in row mode, which means that an operator processes one row at a time. This sounds inefficient, but tends to work out pretty well in practice. However, something which may work out even better is to process more than one row at a time, especially when the number of rows gets to be fairly large. Enter batch mode.
Batch mode was introduced in SQL Server 2012 alongside non-clustered columnstore indexes. It became interesting in SQL Server 2016 and very interesting in SQL Server 2019. That’s because 2016 introduced writable clustered columnstore indexes and 2019 gives us batch mode outside of columnstore indexes.
There are some nice potential performance gains for queries involving window functions.