ActiveRecord query timeout: ScheduleMigrateExternalDiffsWorker

Summary

Observed on gstg: https://sentry.gitlab.net/gitlab/staginggitlabcom/issues/1687958/?query=is%3Aunresolved

PG::QueryCanceled: ERROR:  canceling statement due to statement timeout

  from active_record/connection_adapters/postgresql_adapter.rb:675:in `exec_params'
  # ...
  from active_record/relation/calculations.rb:193:in `pluck'
  from merge_request_diff.rb:109:in `ids_for_external_storage_migration'
  from merge_requests/migrate_external_diffs_service.rb:10:in `enqueue!'
  from schedule_migrate_external_diffs_worker.rb:17:in `block in perform'

It only seems to happen intermittently

Steps to reproduce

1. Have a very large merge_request_diffs table where (almost?) all are migrated
2. Run: MergeRequestDiff.ids_for_external_storage_migration

What is the current bug behavior?

Statement timeout

What is the expected correct behavior?

Query should complete successfully

Output of checks

This bug happens on GitLab.com

Possible fixes

We did carefully index this query. Why it's failing sometimes, but not other times, I'm unsure.