MM-27575: Use index hints to prevent index_merge_intersection (#15207)

Even after specifying all the 3 columns to coerce MySQL into choosing
the right multi-column index, it sometimes uses 2 separate indices
and does an index_merge of them.

This creates problems because the DeleteAt=0 search is essentially
the entire Posts table, and causes a disastrously bad performance
than even choosing the wrong index (idx_create_at).

The problem with this approach is that we hardcode the decision
to a specific index when MySQL was free to choose the right index
depending on table statistics. However, there does not appear to be
a case where this index can cause regressions than using some other index.

Another option here was to set optimizer_switch="index_merge_intersection=off"
at a session level for a transaction and then switch it back on after the
query is done.

However, this can cause some unintentional consequences because this setting
is only available at a session level and not at a query level.
There is no need to set something at the session level when an index hint suffices.

https://mattermost.atlassian.net/browse/MM-27575

store/sqlstore/post_store.go

@@ -708,10 +708,18 @@ func (s *SqlPostStore) getPostsAround(before bool, options model.GetPostsOptions
 		direction = ">"
 		sort = "ASC"
 	}
+	table := "Posts p"
+	// We force MySQL to use the right index to prevent it from accidentally
+	// using the index_merge_intersection optimization.
+	// See MM-27575.
+	if s.DriverName() == model.DATABASE_DRIVER_MYSQL {
+		table += " USE INDEX(idx_posts_channel_id_delete_at_create_at)"
+	}
+
 	replyCountSubQuery := s.getQueryBuilder().Select("COUNT(Posts.Id)").From("Posts").Where(sq.Expr("Posts.RootId = (CASE WHEN p.RootId = '' THEN p.Id ELSE p.RootId END) AND Posts.DeleteAt = 0"))
 	query := s.getQueryBuilder().Select("p.*")
 	query = query.Column(sq.Alias(replyCountSubQuery, "ReplyCount"))
-	query = query.From("Posts p").
+	query = query.From(table).
 		Where(sq.And{
 			sq.Expr(`CreateAt `+direction+` (SELECT CreateAt FROM Posts WHERE Id = ?)`, options.PostId),
 			sq.Eq{"ChannelId": options.ChannelId},
@@ -811,9 +819,17 @@ func (s *SqlPostStore) getPostIdAroundTime(channelId string, time int64, before
 		sort = "ASC"
 	}
 
+	table := "Posts"
+	// We force MySQL to use the right index to prevent it from accidentally
+	// using the index_merge_intersection optimization.
+	// See MM-27575.
+	if s.DriverName() == model.DATABASE_DRIVER_MYSQL {
+		table += " USE INDEX(idx_posts_channel_id_delete_at_create_at)"
+	}
+
 	query := s.getQueryBuilder().
 		Select("Id").
-		From("Posts").
+		From(table).
 		Where(sq.And{
 			direction,
 			sq.Eq{"ChannelId": channelId},
@@ -841,9 +857,17 @@ func (s *SqlPostStore) getPostIdAroundTime(channelId string, time int64, before
 }
 
 func (s *SqlPostStore) GetPostAfterTime(channelId string, time int64) (*model.Post, *model.AppError) {
+	table := "Posts"
+	// We force MySQL to use the right index to prevent it from accidentally
+	// using the index_merge_intersection optimization.
+	// See MM-27575.
+	if s.DriverName() == model.DATABASE_DRIVER_MYSQL {
+		table += " USE INDEX(idx_posts_channel_id_delete_at_create_at)"
+	}
+
 	query := s.getQueryBuilder().
 		Select("*").
-		From("Posts").
+		From(table).
 		Where(sq.And{
 			sq.Gt{"CreateAt": time},
 			sq.Eq{"ChannelId": channelId},