page_len
is queried for various purposes, including Special:ShortPages. It's presumably denormalized from page_latest
ārev_len
to avoid having to join and to allow for the page_len
index on the page table.
rev_len
seems to be used just for displaying the size of the revision (and size change compared to the previous revision) in history pages and such. It may still be useful as the sum of content_len
for that purpose to avoid history pages having to join slots and content and add the lengths together (either with a GROUP BY or in PHP).