Earlier versions accepted DESC in an index definition and ignored it, which is worse than rejecting it because the query looked optimised and was doing a filesort.
-- 5.7 parses this and builds an ascending index
-- 8.0 builds what it says
CREATE INDEX idx_feed ON posts (author_id ASC, published_at DESC);
-- which serves this without a sort
SELECT * FROM posts
WHERE author_id = 4 ORDER BY published_at DESC LIMIT 20;It only matters for a mixed-direction sort — a single-column index has always been readable backwards at no cost. The case it solves is exactly the feed query above, where the leading column is an equality and the second is a descending sort, and before 8.0 that meant either a filesort or storing a negated column. Checking EXPLAIN for Using filesort before and after is the only way to know it helped.