Many procedures in SAS software add the variables from the input data set when an observationwise output data set is created. The assumption of high-performance analytical procedures is that the input data sets can be large and contain many variables. For performance reasons, the output data set contains the following:
variables that are explicitly created by the statement
variables that are listed in the ID statement, as described in Chapter 22, Shared Concepts and Topics in High-Performance Statistical Procedures
distribution keys or hash keys that are transferred from the input data set
Including this information enables you to add to the output data set information necessary for subsequent SQL joins without copying the entire input data set to the output data set.