Refactor sticky order clause into a subquery

Based on some limited testing, using a subquery seems to outperform a
join in this case (the join was invoking a temporary table, which is
always a bad sign).

This also adds logic to fix a bug where sticky discussions would remain
at the top even when marked as read using the "mark all as read" button.
I thought we had an open issue for this somewhere, but I can't seem to
find one. ref #988 #1003

extensions/sticky/src/Listener/PinStickiedDiscussionsToTop.php

@@ -45,10 +45,17 @@ class PinStickiedDiscussionsToTop
             $search = $event->search;
             $query = $search->getQuery();
 
+            // TODO: ideally we might like to consider an event in core that is
+            // fired before the sort criteria is applied to the query (ie.
+            // immediately after gambits are applied) so that we can add the
+            // following order logic to the start without using array_unshift.
+
             if (! is_array($query->orders)) {
                 $query->orders = [];
             }
 
+            // If we are viewing a specific tag, then pin all stickied
+            // discussions to the top no matter what.
             foreach ($search->getActiveGambits() as $gambit) {
                 if ($gambit instanceof TagGambit) {
                     array_unshift($query->orders, ['column' => 'is_sticky', 'direction' => 'desc']);
@@ -57,21 +64,28 @@ class PinStickiedDiscussionsToTop
                 }
             }
 
-            $query->leftJoin('users_discussions', function ($join) use ($search) {
+            // Otherwise, if we are viewing "all discussions" or similar, only
-                $join->on('users_discussions.discussion_id', '=', 'discussions.id')
+            // pin stickied discussions to the top if they are unread. To do
-                     ->where('discussions.is_sticky', '=', true)
+            // this we construct an order clause containing a subquery which
-                     ->where('users_discussions.user_id', '=', $search->getActor()->id);
+            // determines whether or not the discussion is unread.
-            });
+            $subquery = $query->newQuery()
-
+                ->selectRaw(1)
-            // TODO: Might be quicker to do a subquery in the order clause than a join?
+                ->from('users_discussions as sticky')
-            $grammar = $query->getGrammar();
+                ->whereRaw('sticky.discussion_id = discussions.id')
-            $readNumber = $grammar->wrap('users_discussions.read_number');
+                ->where('sticky.user_id', '=', $search->getActor()->id)
-            $lastPostNumber = $grammar->wrap('discussions.last_post_number');
+                ->where(function ($query) {
+                    $query->whereNull('sticky.read_number')
+                        ->orWhereRaw('discussions.last_post_number > sticky.read_number');
+                })
+                ->where('discussions.last_time', '>', $search->getActor()->read_time ?: 0);
 
             array_unshift($query->orders, [
                 'type' => 'raw',
-                'sql' => "(is_sticky AND ($readNumber IS NULL OR $lastPostNumber > $readNumber)) desc"
+                'sql' => "(is_sticky and exists ({$subquery->toSql()})) desc"
             ]);
+
+            $orderBindings = $query->getRawBindings()['order'];
+            $query->setBindings(array_merge($subquery->getBindings(), $orderBindings), 'order');
         }
     }
 }