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