PK and FK using Hash index

On version 10 and 11 hash index was redesigned and now seems to be better
than btree.
Their speed, size and memory consuption are better, so ...
Why is not possible to have all PK and FK just using hash indices ? The only
thing I need on a PK and FK is search and join, so hash index responds very
well.

I know a hash doesn´t accept unique index, so I cannot create a primary key
on it.
But I think it would be great, isn´t it ?

There is something wrong with my thought ?
If not, there is a way to have PK/FH structure using Hash index ?



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