Dear Sergey, While considering the feature once again, I came up with another issue.
When pg_upgrade migrates objects to a new instance, it normally checks whether the new instance does not have existing objects. E.g., check_new_cluster_is_empty() checks if no tables exist, and check_new_cluster_replication_slots() ensures there are no logical slots. Based on that, how should we handle the commit timestamp? Since the patch simply copies the pg_commit_ts subdir, existing entries in the new cluster will be overwritten. So we may have to check if the commit_ts entries are empty when the pg_upgrade command runs, but not sure how to do that. Best regards, Hayato Kuroda FUJITSU LIMITED
