I believe this should raise an error or at least a warning?
postgres=# alter system set storage_engine = 'zheap'; ALTER SYSTEM postgres=# alter system set storage_engine = 'zheap2'; ALTER SYSTEM postgres=# alter system set storage_engine = 'zheap3'; ALTER SYSTEM postgres=#
Sure, if we want we can raise the error or warning for this, but this is a parameter mainly to test zheap with existing set of regression tests. I am not sure if we want to keep it or even if we want to have any such parameter for testing purpose in what form it will be present. So, adding more checks at this stage around this parameter doesn't seem advisable to me.
