BUG #16593: pg_upgrade make corrupt UK indexes

The following bug has been logged on the website:

Bug reference:      16593
Logged by:          László Tóth
Email address:      hali19790320@gmail.com
PostgreSQL version: 9.6.19
Operating system:   centos7
Description:

We upgrade from 9.6 Centos7 to 12.3 Centos 8

1. Stop 9.6
2. rsync 9.6 data and bin files from Centos7 to Centos8 (pre installed 12)
3. pg_upgrade
4. ./analyze_new_cluster.sh
5. ./delete_old_cluster.sh

We found amcheck and throwed exception!

But yes, by amcheck wiki 
https://www.postgresql.org/docs/10/amcheck.html

Whe select is a little bit missunderstandable. I think it will be a little
bit more understandable if write the function is for check one index. So the
query where statement (LIMIT 10 and AND n.nspname = 'pg_catalog' ) ignores a
lot of serious index validation! 

It will be very useful a checklist how can we be sure the upgrade is
succesful!

Thank you!

Here documented it:

https://stackoverflow.com/questions/63588123/postgres-pg-upgrade-corrupt-indexes-uk-missing-values-though-missing-records-b