SAS/ACCESS Software for PC File Formats: Reference

Performance and Efficient View Descriptors

General Guidelines

When you create and use view descriptors, follow these guidelines to minimize the use of SAS System resources and to reduce the time it takes to access data:

Select only the columns your SAS program needs. Selecting unnecessary columns adds extra processing time.
Where possible, specify selection criteria to subset the number of observations processed by the SAS System.
To present PC data in sorted order, reference a view descriptor in a PROC SQL query. Otherwise, you may need to extract the data to sort it, as described below.

Extracting Data Using a View

In some cases, it might be more efficient to use a view descriptor to extract PC file data and place it in a SAS data file instead of using the view descriptor to read the data directly.

A PC file is read every time a view descriptor is referred to in a SAS program and the program is executed; the program's output reflects the latest updated level of the PC file. If many users are reading the same PC file repeatedly, PC file performance may decrease. If you create several reports during the same SAS session, they may not be based on the same PC file data due to updating by other users. Therefore, in the following circumstances, it is better to extract data:

Extract PC file data if the file is large and you use the data repeatedly in SAS programs.
If a view descriptor describes a large PC file and you plan to use the same PC file data in several procedures or DATA steps during the same SAS session, you might improve performance by extracting the data. Placing the data into a SAS data file requires a certain amount of disk space to store the data and I/O to write the data. However, SAS data files are organized to provide optimal performance with PROC and DATA steps. Programs that use SAS data files are often more efficient than programs that read PC file data directly.
Extract PC file data if you use sorted data several times in a SAS program.
If you intend to use PC file data in a particular sorted order several times, run the SORT procedure on the view descriptor using the OUT= option to extract the data. The OUT= option is required whenever PROC SORT references a view descriptor. Extracting the data in this way is more efficient than requesting the same sort repeatedly on the PC file data. Note that using the ORDER BY clause in the SQL procedure does not sort the data in the physical PC file; it only presents the data in a sorted order.
Extract PC file data for added security.
If you are the owner of a PC file and do not want anyone else to read the data, you might want to extract the data (or a subset of the data) and not distribute information about either the access descriptor or view descriptor. Or, you might want to assign PC file security features to your PC files to prevent unauthorized reading or writing to them.
On the SAS System side, you might also want to assign SAS System passwords to your descriptors for additional security. If a view descriptor has a password assigned to it and you extract the data, the new SAS data file is automatically assigned the same password. If a view descriptor does not have a password, you can assign a password to the extracted SAS data file.

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