Data that you cannot compress
You cannot compress data in rows in some types of tables and fragments.
- Tables or fragments that are in the sysmaster, sysutils, sysuser, syscdr, and syscdcv1 databases
- Catalogs
- Temporary tables
- Virtual-table interface tables
- The tblspace tblspace
- Internal partition tables
- Dictionary tables (these tables, one per dbspace, hold compression dictionaries for the fragments or tables that are compressed in that dbspace and metadata about the dictionaries.)
You cannot compress a table while an online index build is occurring on the table.
You cannot compress simple large objects in blobspaces.
Encrypted data, data that is already compressed by another algorithm, and data without long repeating patterns compresses poorly or does not compress. Try to avoid placing columns with data that compresses poorly between columns that have frequent patterns to prevent the potential disruption of column-spanning patterns.
IBM® Informix® compresses images of the rows only if the images of the compressed rows are smaller than the uncompressed images. Even if compressed rows are only slightly smaller than their uncompressed images, a small saving of space can enable the server to put more rows onto pages.
Very small tables are not good candidates for compression, because you might not be able to gain back enough space from compressing the rows to offset the storage cost of the compression dictionary.
Informix cannot compress an individual row to be smaller than four bytes long. The server must leave room in case the row image later grows beyond what the page can hold. Therefore, you must not try to compress fragments or non-fragmented tables with rows that contain four bytes or are shorter than four bytes.