s4 today was doing ~6.5M rps of reads for ~20 minutes, causing database lag and "database locked" errors on attempting to use uploadwizard.
The query at fault looks to be:
UPDATE /* WikiPage::updateCategoryCounts */ `category` SET cat_pages = cat_pages - 1, cat_subcats = cat_subcats - 1 WHERE cat_title = 11
This should probably instead be WHERE cat_title = '11'. This bug is possibly similar to T239877 ?