Speed up Open list when there are assignee lists

When there is an assignee list and the board can show a lot of issues,
the Open list can be very slow. This was because the query was
essentially repeated inside a NOT IN to exclude issues that are assigned
to someone from one of the assignee lists.

Instead, we can simplify the inner query to be a more direct one:

    NOT EXISTS (
      SELECT 1
      FROM issue_assignees
      WHERE user_id IN ($assignee_list_users)
        AND issue_id = issues.id
    )

Which also performs much better, as well as being simpler to understand.

ee/app/services/ee/boards/issues/list_service.rb

@@ -60,8 +60,12 @@ module EE
         def without_assignees_from_lists(issues)
           return issues if all_assignee_lists.empty?
 
-          issues
-            .where.not(id: issues.joins(:assignees).where(users: { id: all_assignee_lists.select(:user_id) }))
+          matching_assignee = ::IssueAssignee
+                                .where(user_id: all_assignee_lists.reorder(nil).select(:user_id))
+                                .where("issue_id = issues.id")
+                                .select(1)
+
+          issues.where('NOT EXISTS (?)', matching_assignee)
         end
         # rubocop: enable CodeReuse/ActiveRecord
 

ee/changelogs/unreleased/speed-up-open-list-for-boards-with-assignee-lists.yml

+---
+title: Fix timeout loading Open list when board contains assignee lists
+merge_request:
+author:
+type: performance