> ALTER ROLE "MyRole" set default_transaction_read_only = on;
> Because I wanted this role to readonly (can not change anything in DB but > only view).
You realize, I hope, that breaking out of that restriction is no harder than issuing
SET default_transaction_read_only = off;
or even
BEGIN TRANSACTION READ WRITE;
So that ALTER ROLE might be of some use as a protection against accidental changes, but it's certainly no form of security restriction. (What you probably want to do instead of this is make sure the role doesn't have select/update/delete privileges for any of your tables.)
> But later I realized this role is not even allowed to change his password.