Revert "MM-9770: rewrite getParentsPosts to improve performance (#8467)" (#8659) (#8694)

This reverts commit 4b675b347b.

store/sqlstore/post_store.go

@@ -715,70 +715,31 @@ func (s *SqlPostStore) getRootPosts(channelId string, offset int, limit int) sto
 func (s *SqlPostStore) getParentsPosts(channelId string, offset int, limit int) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		var posts []*model.Post
-		_, err := s.GetReplica().Select(&posts, `
-			SELECT
-			    *
+		_, err := s.GetReplica().Select(&posts,
+			`SELECT
+			    q2.*
 			FROM
-			    Posts
+			    Posts q2
+			        INNER JOIN
+			    (SELECT DISTINCT
+			        q3.RootId
+			    FROM
+			        (SELECT
+			        RootId
+			    FROM
+			        Posts
+			    WHERE
+			        ChannelId = :ChannelId1
+			            AND DeleteAt = 0
+			    ORDER BY CreateAt DESC
+			    LIMIT :Limit OFFSET :Offset) q3
+			    WHERE q3.RootId != '') q1
+			    ON q1.RootId = q2.Id OR q1.RootId = q2.RootId
 			WHERE
-			    Id IN (SELECT * FROM (
-				-- The root post of any replies in the window
-				(SELECT * FROM (
-				    SELECT
-					CASE RootId
-					    WHEN '' THEN NULL
-					    ELSE RootId
-					END
-				    FROM
-					Posts
-				    WHERE
-					ChannelId = :ChannelId1
-				    AND DeleteAt = 0
-				    ORDER BY 
-					CreateAt DESC
-				    LIMIT :Limit1 OFFSET :Offset1
-				) x )
-
-				UNION
-
-				-- The reply posts to all threads intersecting with the window, including replies
-				-- to root posts in the window itself.
-				(
-				    SELECT
-					Id
-				    FROM
-					Posts
-				    WHERE RootId IN (SELECT * FROM (
-					SELECT
-					    CASE RootId
-						-- If there is no RootId, return the post id itself to be considered
-						-- as a root post.
-						WHEN '' THEN Id
-						-- If there is a RootId, this post isn't a root post and return its
-						-- root to be considered as a root post.
-						ELSE RootId
-					    END
-					FROM
-					    Posts
-					WHERE
-					    ChannelId = :ChannelId2
-					AND DeleteAt = 0
-					ORDER BY 
-					    CreateAt DESC
-					LIMIT :Limit2 OFFSET :Offset2
-				    ) x )
-				)
-			    ) x )
-			AND 
-			    DeleteAt = 0
-		`, map[string]interface{}{
-			"ChannelId1": channelId,
-			"ChannelId2": channelId,
-			"Offset1":    offset,
-			"Offset2":    offset,
-			"Limit1":     limit,
-			"Limit2":     limit,
-		})
+			    ChannelId = :ChannelId2
+			        AND DeleteAt = 0
+			ORDER BY CreateAt`,
+			map[string]interface{}{"ChannelId1": channelId, "Offset": offset, "Limit": limit, "ChannelId2": channelId})
 		if err != nil {
 			result.Err = model.NewAppError("SqlPostStore.GetLinearPosts", "store.sql_post.get_parents_posts.app_error", nil, "channelId="+channelId+" err="+err.Error(), http.StatusInternalServerError)
 		} else {