When
using an index to select a subset, SAS loads only the pages that contain
at least one qualified observation into input buffers. When accessing
observations sequentially, SAS must load all pages into input buffers.
Loading more pages requires more I/O operations.
Correct answer: b
SAS
does not use an index for a WHERE condition that contains a function
other than TRIM or SUBSTR.
Correct answer: a
The size of the subset
relative to the size of the data set is an important factor in determining
which access method is most efficient. If a subset is large (more
than 33% of the data set), it is likely to be more efficient to use
sequential access than direct access. Direct access is usually more
efficient when you select a small subset (less than 33% of the data
set), especially if the data set is large (has a high page count).
However, if the data set is very small (less than three pages), using
an index is not efficient. The number of key variables specified in
a WHERE expression does not determine which access method is most
efficient. If the two key variables that are specified are the first
two variables in the same index, the WHERE expression is a candidate
for compound optimization. Sorting the data also does not determine
which access method is most efficient. However, sorting the data before
subsetting improves the efficiency of WHERE processing regardless
of the access method.