The overhead associated with a compressed data set is 12 or 24 bytes per observation for 32-bit or 64-bit hosts, respectively. Because of the additional overhead, it is possible for a compressed data set to be larger than an uncompressed version. (The notes written to the SAS Log indicating the effectiveness of the compression should be used during application development to determine if data set compression offers any benefits, given the specific structure and content of the data set(s) involved.)

SAS compression is most useful for data sets with long observation lengths and/or many repeating characters (such as blanks). When a request is made to compress a data set, SAS attempts to determine if compression will increase the size of the data set by reading the header information and examining the types and lengths of the variables. If, due to the number and type of the variables, it is not possible for the compressed data set to be at least 12 or 24 bytes (for 32- and 64-bit hosts, respectively) per observation smaller, compression is disabled and a message is written to the SAS log.

In the following example, SAS determines that it is not possible for the compressed data set to be smaller than an uncompressed one:

   data one (compress=yes);
     length x y $2;
     input x y;
   datalines;
   ab cd
   ;
   run;

The following message is written to the SAS log: NOTE: Compression was disabled for data set WORK.ONE because compression overhead would increase the size of the data set. Prior to SAS 9.1, this message is issued as a WARNING.

Operating System and Release Information

Product Family	Product	System	SAS Release
			Reported	Fixed*
SAS System	Base SAS	Microsoft® Windows® for 64-Bit Itanium-based Systems	9.1 TS1M0
		Microsoft Windows 2000 Professional	9.1 TS1M0
		Microsoft Windows NT Workstation	9.1 TS1M0
		Microsoft Windows 2000 Server	9.1 TS1M0
		Microsoft Windows 2000 Advanced Server	9.1 TS1M0
		Microsoft Windows 2000 Datacenter Server	9.1 TS1M0
		HP-UX IPF	9.1 TS1M0
		Linux	9.1 TS1M0
		z/OS	9.1 TS1M0
		64-bit Enabled Solaris	9.1 TS1M0
		64-bit Enabled AIX	9.1 TS1M0
		OpenVMS Alpha	9.1 TS1M0
		64-bit Enabled HP-UX	9.1 TS1M0
		Tru64 UNIX	9.1 TS1M0
		Microsoft Windows XP Professional	9.1 TS1M0

* For software releases that are not yet generally available, the Fixed Release is the software release in which the problem is planned to be fixed.

---

Type:	Usage Note
Priority:
Topic:	Data Management ==> Access ==> SAS I/O Data Management ==> Manipulation and Transformation ==> Compressing Data Files

Date Modified:	2008-03-28 10:41:17
Date Created:	2002-10-09 12:10:46