Improve performance of group work items query
What does this MR do and why?
Use a UNION instead of UNION ALL because it results in a much better query plan.
This is behind the namespace_level_work_items feature flag. This is an alpha feature in active development.
This fixes the timeout that happens in https://gitlab.com/gitlab-org/plan-stage.
- Previous query: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/25418/commands/80532
- New query: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/25418/commands/80510
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Edited by Heinrich Lee Yu