Order of the Data
|
Effect on I/O Operations
|
---|---|
observations are randomly
distributed throughout the data file
|
Qualified
observations are located on a larger number of data file pages. An
I/O operation is required each time that SAS loads a page. Therefore,
the more random the data in the data file, the more I/O operations
are needed to use the index.
|
observations are sorted
on the indexed variable or variables
|
The
data is ordered more like the index (in ascending value order), so
qualified observations are located on fewer data file pages. Therefore,
the less random the data in the data file, the fewer I/O operations
are needed to use the index.
|