With the "runuser -u postgres -- reindexdb -a " step, one can run into two different issues:

A) With rhsm related facts


# runuser -u postgres -- reindexdb -a 
could not change directory to "/root": Permission denied 
reindexdb: reindexing database "candlepin" 
reindexdb: reindexing database "foreman" 
reindexdb: error: reindexing of database "foreman" failed: ERROR: could not create unique index "index_fact_names_on_name_and_type" DETAIL: Key (name, type)=(memory::swaptotal, Katello::RhsmFactName) is duplicated.


To fix the same, Delete any facts that are of type 'Katello::RhsmFactName' 

# cat << EOF | foreman-rake console
conf.echo = false
fact_name_ids = FactName.unscoped.where(:type => 'Katello::RhsmFactName').ids
pp fact_name_ids.count
FactValue.unscoped.where(:fact_name_id => fact_name_ids).delete_all
FactName.unscoped.where(:id => fact_name_ids).delete_all
pp FactName.unscoped.where(:type => 'Katello::RhsmFactName').ids.count
EOF


These facts will come back on their own when the rhsm checkin's will happen for the individual client systems ( in every 4 hours ). 



B) With puppet related facts :

# runuser -u postgres -- reindexdb -a 
could not change directory to "/root": Permission denied 
reindexdb: reindexing database "candlepin" 
reindexdb: reindexing database "foreman" 
reindexdb: error: reindexing of database "foreman" failed: ERROR:  could not create unique index "index_fact_names_on_name_and_type"
DETAIL:  Key (name, type)=(augeasversion, PuppetFactName) is duplicated.


To fix the same, Delete any facts that are of type 'PuppetFactName' 

# cat << EOF | foreman-rake console
conf.echo = false
fact_name_ids = FactName.unscoped.where(:type => 'PuppetFactName').ids
pp fact_name_ids.count
FactValue.unscoped.where(:fact_name_id => fact_name_ids).delete_all
FactName.unscoped.where(:id => fact_name_ids).delete_all
pp FactName.unscoped.where(:type => 'PuppetFactName').ids.count
EOF


These facts will also come back on their own when puppet agent will be executed on concerned systems or a scheduled puppet run will happen on the client systems.


Once done, Retry the reindexing 

# runuser -u postgres -- reindexdb -a


Once it is successful and all services of satellite are running fine, Proceed with executing the post-upgrade steps from https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/upgrading_from_rhel_7_to_rhel_8/performing-post-upgrade-tasks-rhel-7-to-rhel-8_upgrading-from-rhel-7-to-rhel-8#doc-wrapper

We have another situation where the facts are not affected but it's katello_available_module_streams 

# runuser -u postgres -- reindexdb -a
reindexdb: reindexing database "candlepin"
reindexdb: reindexing database "foreman"
reindexdb: error: reindexing of database "foreman" failed: ERROR: could not create unique index "katello_available_module_streams_name_stream_context".
DETAIL: key (name, stream, context)=(pmdk, 1-fileformat-v6, b4937e53) is duplicated.

KB:

Reindexing of foreman database fails with error `ERROR: could not create unique index "katello_available_module_streams_name_stream_context"` after the in-place OS upgrade of Satellite 6.11 to RHEL 8 - Red Hat Customer Portal
https://access.redhat.com/solutions/7004750