[PR #8135/b3c4388d backport][3.119] Improve the performance of the latest publication lookup when serving content from a repository-backed distribution.

[patchback]

This is a backport of PR #8135 as merged into main (b3c4388).

Distribution.get_repository_publication_and_version() resolves the newest publication for a repository-backed distribution with repository_version__in=repository.versions.all(). Django renders that as a subquery joined on pulp_id, which hides repository_id from the planner. It cannot use the (repository_id, number) unique index and instead scans the global index on number, filtering each row by repository.

Since number is a per-repository counter and that index is global, the work is set by the total number of repository versions across all repositories rather than by the size of the repository being served. On an installation with a large number of repository versions this scans a substantial portion of core_repositoryversion to return a single row, and every repository degrades as any of them grow.

Filtering with repository_version__repository selects the same rows but keeps repository_id visible, so the planner uses the (repository_id, number) index and the lookup becomes a short index scan.

The same query was also hydrating repository_version.content_ids through select_related, which nothing on this path reads; it is now deferred.

closes #1995

Assisted-by: Claude Opus 5 (GitHub Copilot)

📜 Checklist

• Commits are cleanly separated with meaningful messages (simple features and bug fixes should be squashed to one commit)
• A changelog entry or entries has been added for any significant changes
• Follows the Pulp policy on AI Usage
• (For new features) - User documentation and test coverage has been added

See: Pull Request Walkthrough