Estimating the Number of Observations
It is more efficient to use indexed access for a small subset and to use sequential access for a large subset. Therefore, after identifying any available indexes and evaluating the conditions in the WHERE expression, SAS estimates the number of observations that will be qualified by the index. Whether SAS uses an index depends on the percentage of observations that are qualified (the size of the subset relative to the size of the data set), as shown below:
If the subset is less than 3% of the data set, direct access is almost certainly more efficient than sequential access, and SAS will use ...