Toby Zerner 0ce00c6d16 Performance: Refactor SQL that pins unread sticky posts to the top ... Ordering by `is_sticky and (unread subquery) desc` removes MySQL's ability to use an index for ordering by `last_time`, which triggers a filesort across the whole discussions table which is BAD. This commit uses a union to add all stickied discussions to the query. The results of the unioned queries are then ordered by the `is_sticky and (unread subquery)` criteria, so the filesort only takes place on a maximum of limit * 2 rows. Big performance win when you get up to thousands of discussions!		2017-11-12 21:28:17 +10:30
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sticky	Performance: Refactor SQL that pins unread sticky posts to the top	2017-11-12 21:28:17 +10:30