Postgres hash index with unique constraint

Question

With Postgres 10 properly supporting hash index, I would like to use hash index for id lookup (hash index is smaller in size compared to btree and theoretically faster).

I have a table

create table t (id int);
create unique index on t using hash (id);

But I got the following:

ERROR: access method "hash" does not support unique indexes

Why does hash index not allow unique constraint? Are there ways to circumvent this?

Answer 1

The documentation leaves no room for doubt:

Currently, only B-tree indexes can be declared unique.

There was a discussion on the hackers list about this recently, and it was concluded that it wouldn't be simple to add the capability to allow UNIQUE hash indexes.

Answer 2

You can achieve this using an exclusion constraint:

create table t (id int);
alter table t add constraint c exclude using hash (id with =);