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The following query is much slower on MariaDB enwiki with the FORCE INDEX than
without it:

SELECT /* IndexPager::buildQueryInfo (NewPagesPager) xxx.xxx.xxx.xxx */ rc_namespace, rc_title, rc_cur_id, rc_user, rc_user_text, rc_comment, rc_timestamp, rc_patrolled, rc_id, rc_deleted, page_len AS length, page_latest AS rev_id, rc_this_oldid, page_namespace, page_title, fp_stable, fp_pending_since, (SELECT GROUP_CONCAT(ct_tag SEPARATOR ', ') FROM `change_tag` WHERE ct_rc_id=rc_id ) AS ts_tags FROM `recentchanges` FORCE INDEX (new_name_timestamp) INNER JOIN `page` ON ((page_id=rc_cur_id)) LEFT JOIN `flaggedpages` ON ((fp_page_id = rc_cur_id)) WHERE rc_new = '1' AND (rc_namespace != '6') AND page_is_redirect = '0' ORDER BY rc_timestamp DESC LIMIT 51;

With force: > 1min, ~500000 rows hit with a filesort.

Without force: < 1ses, ~100 rows hit using rc_timestamp, no filesort.