Resolve "Slow SQL running in production queryid 3257994922"

What does this MR do?

Previous query

SELECT "projects".*
FROM "projects"
INNER JOIN "namespaces" ON "namespaces"."id" = "projects"."namespace_id"
WHERE (
  namespace_id IN (
   SELECT "namespaces"."id"
   FROM "namespaces"
   INNER JOIN "members" ON "namespaces"."id" = "members"."source_id"
   WHERE "members"."source_type" = 'Namespace'
    AND "namespaces"."type" IN ('Group')
    AND "members"."user_id" = 1
    AND "namespaces"."type" IN ('Group')
    AND "members"."access_level" = 50
    AND "members"."type" IN ('GroupMember')
    AND "members"."requested_at" IS NULL
   )
  OR namespace_id = 2
)

New query

SELECT
  "projects".*
FROM (SELECT
  "projects".*
FROM "projects"
WHERE "projects"."namespace_id" = 752513
UNION ALL
SELECT
  "projects".*
FROM "projects"
INNER JOIN "project_authorizations"
  ON "project_authorizations"."project_id" = "projects"."id"
WHERE (projects.namespace_id <> 752513)
AND "project_authorizations"."user_id" = 632546
AND "project_authorizations"."access_level" = 50) AS projects

Improves the performance of the query in https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/models/user.rb#L625

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #43552 (closed)