Segby is a specific segmentation where the values in the segby columns govern the way the rows are split. If a table is segby a given column, no unique value of the column can be found in more than one segment. This allows for quick computation of aggregate/grouping functions (e.g., sums, averages, etc.) on that column since it is only necessary to look in one file for each unique value of that column. To achieve this, the rows of the table frequently need to be reordered.