Details on page compression (page-dictionary)

(Sunil Agarwal) In the previous blog, I described column-prefix compression that is done as part of PAGE compression. The other component of PAGE compression is the page-dictionary. As the name indicates, this dictionary is specific to a page and the page can be data page or a leaf-index page. The non-leaf pages of an index are ROW compressed even if you choose to compress the index with PAGE compression option. It was a design decision to reduce the CPU impact (it is more expensive to decompress a row with PAGE compression compared to ROW compression) of index tree traversal but on the flip side the height of the index tree could be smaller depending on the data in the index key. Now back to page-dictionary, the topic of this blog.