Software-based BLC is a space-saving feature and rarely helps performance. The following table shows examples of relative CPU cost per MB to compress or decompress data, assuming a large table and full-table scans. Increasing the amount of joins and indexes, or the retrieval spool used by executing queries, reduces the amount of compression I/O.
| Platform | CPU Cost (CPU msec/MB) |
Uncompressed I/O (MB/sec/node) |
Compressed I/O (MB/sec/node) |
|||
|---|---|---|---|---|---|---|
| Compress | Uncompress | Compress | Uncompress | Compress | Uncompress | |
| 2500/5500 | 50.0 - 100.0 | 7.0 - 14.0 | 40 - 80 | 364 - 571 | 13 - 27 | 121 - 190 |
| 1550/2550/5550 | 63.9 - 127.8 | 8.9 - 14.1 | 63 -125 | 569 - 894 | 21 - 42 | 190 - 298 |
| 1600/2580/5600 | 73.8 - 147.5 | 10.3 - 16.2 | 108 - 217 | 986 - 1550 | 36 - 72 | 329 - 517 |
| 2650/5650 | 73.8 - 147.5 | 10.3 -16.2 | 163 - 325 | 1479 - 2324 | 54 - 108 | 493 - 775 |