SAS 9.4 Language Reference, 6th Edition

observations, and whether other engines are required for processing). The engine

uses this information to organize the data in the standard logical form for SAS

processing.

• This standard form is called the SAS data file, which consists of the descriptor

information and the data values organized into columns (variables) and rows

(observations).

• SAS procedures and DATA step statements access and process the data only in its

logical form. During processing, the engine executes whatever instructions are

necessary to open and close physical files and to read and write data in appropriate

formats.

Data that is accessed by an engine is organized into the SAS data set model, and in the

same way, groups of files that are accessed by an engine are organized in the correct

logical form for SAS processing. Once files are accessed as a SAS library, you can use

SAS utility windows and procedures to list their contents and to manage them. See

Chapter 26, “SAS Libraries,” on page 585 for more information about SAS libraries.

The following figure shows the relationship of engines to SAS libraries.

Figure 37.2 Relationship of Engines to SAS Libraries

files

engine

SAS utility

windows and procedures

SAS data library model

Engine Characteristics

About Engine Characteristics

The engine that is used to access a SAS data set determines its processing characteristics.

Different statements and procedures require different processing characteristics. For

example, the FSEDIT procedure requires the ability to update selected data values. And,

the POINT= option in the SET statement requires random access to observations as well

as the ability to calculate observation numbers from record identifiers within the file.

The following figure describes the types of activities that engines regulate.

Engine Characteristics 747

Figure 37.3 Activities That Engines Regulate

ACCESS

PATTERNS

Engine

READ/WRITE

ACTIVITY

LOCKING

LEVELS

INDEXING

INTEGRITY

CONSTRAINTS

COMPRESSION/REUSE

GENERATIONS

DATA COMPATIBILITY

Cross Platform

Cross Release

Read/Write Activity

An engine can perform one or more of the following tasks:

• limit read/write activity for a SAS data set to read-only

• fully support updating, deleting, renaming, or redefining the attributes of the data set

and its variables

• support only some of these functions

For example, the engines that process BMDP, OSIRIS, or SPSS files support read-only

processing. Some engines that process SAS views permit SAS procedures to modify

existing observations while others do not.

Access Patterns

SAS procedures and statements can read observations in SAS data sets in one of four

general patterns:

sequential access

processes observations one after the other, starting at the beginning of the file and

continuing in sequence to the end of the file.

random access

processes observations according to the value of some indicator variable without

processing previous observations.

BY-group access

groups and processes observations in order of the values of the variables that are

specified in a BY statement.

multiple-pass

performs two or more passes on data when required by SAS statements or

procedures.

If a SAS statement or procedure tries to access a SAS data set whose engine does not

support the required access pattern, SAS prints an appropriate error message in the SAS

log.

Levels of Locking

Some features of SAS require that data sets support different levels at which Update

access is used. When a SAS data set can be opened concurrently by more than one SAS

748 Chapter 37 • SAS Engines