Hi,
After further investigation, it looks to be related to the new DNF
module feature in EL8.
Workaround is to disable the postgresql module:
dnf module disable postgresql
After which, postgresql (ol8_appstream), postgresql10 (pgdg10) and
postgresql11 (pgdg11) are installable.
Still need to look further into how these modules work and if any
packaging changes could be done as a better fix for PGDG.
Andrew