When reindexing deleted pages for CirrusSearch extension, we've discovered that archive table only has one index involving timestamp:
KEY `name_title_timestamp` (`ar_namespace`,`ar_title`,`ar_timestamp`),
This means if we want to reindex deletes from certain timestamp (such as --from option used in in-place reindexing) we can not use index, since there's no index starting with timestamp. It also makes table scan (when reindexing whole set of deleted pages) - which now follows the index - to go in somewhat unnatural order, scanning in title order and not in timestamp order, which makes it hard to track progress.
It would be great if we could add either index on just ar_timestamp or:
KEY `timestamp_name_title` (`ar_timestamp`, `ar_namespace`,`ar_title`),