BRIN indexes and ORDER BY

We're curious about the current behavior in 9.5.4, and possible future enhancements, of BRIN indexes with respect to ordering.

In the docs, section 11.4. "Indexes and ORDER BY" (https://www.postgresql.org/docs/9.5/static/indexes-ordering.html) is clear that anything other than B-tree indexes have unspecified ordering:

"In addition to simply finding the rows to be returned by a query, an index may be able to deliver them in a specific sorted order. This allows a query's ORDER BY specification to be honored without a separate sorting step. Of the index types currently supported by PostgreSQL, only B-tree can produce sorted output — the other index types return matching rows in an unspecified, implementation-dependent order."

We found a pgsql-hackers thread from about a year ago about optimizing ORDER BY for BRIN indexes. Tom Lane suggested that he was working on it: https://www.postgresql.org/message-id/11881.1443393360%40sss.pgh.pa.us

Our current test shows that ordering by a BRIN indexed column still performs an unoptimized sort:

SELECT generate_series(1, 10000000) AS id INTO test;

CREATE INDEX idx_test_id ON test USING BRIN (id);

EXPLAIN SELECT id FROM test ORDER BY id DESC LIMIT 20;

Limit  (cost=410344.40..410344.45 rows=20 width=4)

  ->  Sort  (cost=410344.40..435344.40 rows=1000000 width=4)"

        Sort Key: id DESC

        ->  Seq Scan on test  (cost=0.00..144248.00 rows=10000000 width=4)

Is there anything we're missing to speed this up? Or is it still a future feature?

Thank you,

Darren Lafreniere