How to create INOUT assignment cast on Amazon RDS PostgreSQL? If not possible, then why?
On AWS RDS you must first alter type &lt;type&gt; owner to &lt;user&gt; before create cast since the default owner of all types is the rdsadmin user.

To run alter type you must do so with the master user system account you created for RDS or any other user that you have given the rds_superuser role. From the docs:


  When you create a DB instance, the master user system account that you
  create is assigned to the rds_superuserrole.
  The rds_superuser role is similar to the PostgreSQL superuser role
  (customarily named postgres in local instances) but with some
  restrictions.