SQL Performance tuning (#952)

MySQL has problems with executing this subquery efficiently.

framework/core/src/Core/Repository/PostRepository.php

@@ -146,9 +146,11 @@ class PostRepository
 
     protected function getDiscussionsForPosts($postIds, User $actor)
     {
-        return Discussion::whereIn('id', function ($query) use ($postIds) {
-            $query->select('discussion_id')->from('posts')->whereIn('id', $postIds);
-        })
+        return Discussion::query()
+            ->select('discussions.*')
+            ->join('posts', 'posts.discussion_id', '=', 'discussions.id')
+            ->whereIn('posts.id', $postIds)
+            ->groupBy('discussions.id')
             ->whereVisibleTo($actor)
             ->get();
     }