HCC Compression Unit Sizing
Since the beginning Oracle has provided four compression levels to offer a trade-off between the compression ratio and various other factors including table scans and the performance of single-row retrieval. I can not emphasize enough that the various trade offs mean that YMMV with the different levels and you should always test what works best with your application and hardware when choosing the level. Historically people have rarely used Query Low since the fast compression with reduced compression ratio means that the extra disk I/O is slower than the cost of decompression with Query High. The one time that Query Low makes sense on spinning disks is if you still have a significant number of row retrieval operations (including from index access joins).
X5 introduced NVMe technology which means that the extra I/O from Query Low is faster than ZLIB decompression which makes Query Low beneficial. So we needed to reassess the sizing of Compression Units. from 220.127.116.11 to 18.104.22.168 the sizing guidelines are as follows:Continue reading