Data that you cannot compress

You cannot compress a table while an online index build is occurring on the table.

You cannot compress simple large objects in blobspaces.

You cannot compress indexes.

Compression is not applied to index data, large object data that is stored outside of the row, or any other form of non-row data.

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.

If XML data is stored with the first portion in a row and the remainder outside of the row, compression is only applied to the portion that is stored in the row.

HCL OneDB™ 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.

HCL OneDB 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.