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

SELECT /* SpecialRecentChanges::doMainQuery */  rc_id,rc_timestamp,rc_cur_time,rc_user,rc_user_text,rc_namespace,rc_title,rc_comment,rc_minor,rc_bot,rc_new,rc_cur_id,rc_this_oldid,rc_last_oldid,rc_type,rc_source,rc_patrolled,rc_ip,rc_old_len,rc_new_len,rc_deleted,rc_logid,rc_log_type,rc_log_action,rc_params,wl_user,wl_notificationtimestamp,ts_tags,fp_stable,fp_pending_since  FROM `recentchanges` FORCE INDEX (rc_timestamp) LEFT JOIN `watchlist` ON (wl_user = '12744321' AND (wl_title=rc_title) AND (wl_namespace=rc_namespace)) LEFT JOIN `tag_summary` ON ((ts_rc_id=rc_id)) LEFT JOIN `flaggedpages` ON ((fp_page_id = rc_cur_id))  WHERE (rc_timestamp >= '20131018000000') AND rc_bot = '0' AND (rc_namespace = '828') AND (rc_type != 5) AND rc_new IN ('0','1')   ORDER BY rc_timestamp DESC LIMIT 500

With force: > 1 minute, a slow range scan on rc_timestamp, ~2 million rows.

Without force: < 3 seconds, a fast ref lookup on rc_namespace_title with cheap filesort, ~1000 rows.