|SAS Data Set Options|
|Valid in:||DATA step and PROC steps|
|Category:||Data Set Control|
|Restriction:||Use with output data sets only.|
|COMPRESS=NO | YES | CHAR | BINARY|
specifies that the observations in a newly created SAS data set are compressed (variable-length records) by SAS using RLE (Run Length Encoding). RLE compresses observations by reducing repeated consecutive characters (including blanks) to two-byte or three-byte representations.
|Tip:||Use this compression algorithm for character data.|
Note: COMPRESS=CHAR is accepted by Version 7 and later versions.
specifies that the observations in a newly created SAS data set are compressed (variable-length records) by SAS using RDC (Ross Data Compression). RDC combines run-length encoding and sliding-window compression to compress the file.
|Tip:||This method is highly effective for compressing medium to large (several hundred bytes or larger) blocks of binary data (numeric variables). Because the compression function operates on a single record at a time, the record length needs to be several hundred bytes or larger for effective compression.|
Compressing a file is a process that reduces the number of bytes required to represent each observation. Advantages of compressing a file include reduced storage requirements for the file and fewer I/O operations necessary to read or write to the data during processing. However, more CPU resources are required to read a compressed file (because of the overhead of uncompressing each observation), and there are situations where the resulting file size might increase rather than decrease.
Use the COMPRESS= data set option to compress an individual file. Specify the option for output data sets only--that is, data sets named in the DATA statement of a DATA step or in the OUT= option of a SAS procedure. Use the COMPRESS= data set option only when you are creating a SAS data file (member type DATA). You cannot compress SAS views, because they contain no data.
After a file is compressed, the setting is a permanent attribute of the file, which means that to change the setting, you must re-create the file. That is, to uncompress a file, specify COMPRESS=NO for a DATA step that copies the compressed file.
The data set option POINTOBS=YES, which is the default, determines that a compressed data set can be processed with random access (by observation number) rather than sequential access. With random access, you can specify an observation number in the FSEDIT procedure and the POINT= option in the SET and MODIFY statements.
When you create a compressed file, you can also specify REUSE=YES (as a data set option or system option) in order to track and reuse space. With REUSE=YES, new observations are inserted in space freed when other observations are updated or deleted. When the default REUSE=NO is in effect, new observations are appended to the existing file.
POINTOBS=YES and REUSE=YES are mutually exclusive--that is, they cannot be used together. REUSE=YES takes precedence over POINTOBS=YES. That is, if you set REUSE=YES, SAS automatically sets POINTOBS=NO.
Compressing Data Files in SAS Language Reference: Concepts