Do BRIN indexes support ENUM types?

Question

BRIN indexes seem useful, but I'm not sure how to use one on an ENUM type. I thought this code would work:

CREATE TYPE test_enum AS ENUM ('a', 'b');
CREATE TEMPORARY TABLE my_table (
    x test_enum
);

CREATE INDEX test_index ON my_table using brin (x);
ERROR:  data type test_enum has no default operator class for access method "brin"

Do I have to create a new operator class from scratch? Aren't enums already ordered?

This commit, from 2014, implies that BRIN indexes should work for ENUM types.

Answer 1

Aren't enums already ordered?

No.

This commit, from 2014, implies that BRIN indexes should work for ENUM types.

That's actually not what that commit says. From the commit-message on the link you provided

This type of operator class we call "Minmax", and we supply a bunch of them for most data types with B-tree opclasses. Since the BRIN code is generalized, other approaches are possible for things such as arrays, geometric types, ranges, etc; even for things such as enum types we could do something different than minmax with better results. In this commit I only include minmax.

That doesn't mean it's there now. It is PostgreSQL for "patches accepted." In fact that commit explicitly says,

+/* no brin opclass for enum, tsvector, tsquery, jsonb, range */

Actually creating a BRIN index for an ENUM is, afaik, going to require some use of C and knowledge of index operator class implementation (pick/split/insert/merge).