SAS Certification Prep Guide, 4th Edition

Comparing Probable Resource Usage

Overview

After SAS estimates the number of qualified observations and selects the index that qualifies the fewest observations, SAS must then determine whether it is faster (more efficient) to satisfy the WHERE expression by using the index or by reading all of the observations sequentially. Specifically, SAS predicts how many I/O operations are required in order to satisfy the WHERE expression for each of the access methods. Then it compares the two resource costs.

Note: Remember, if SAS estimates that a subset contains fewer than 3% of the observations in the data set, SAS does not need to estimate resource usage. In this situation, SAS uses the index to process the WHERE statement.

How SAS Compares Resource ...