Comparing Probable Resource Usage
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 ...