Index scan is not pushed down to union all subquery

Hi,

Is it intended that indexes are not pushed down to union all
subqueries if even a single select contains a where clause? Is this
just not implemented, is it impossible to implement or am I doing
something wrong?

The following query does a SeqScan for "bikes" and "cars" tables even
though IndexScan on their column "dealer_name" would be a magnitude
faster. (Schema with sample data at the bottom of this mail)

WITH
targets as (
select 'bike' vehicle, id, dealer_name FROM bikes WHERE frame_size = 52
union all
select 'car' vehicle, id, dealer_name FROM cars
-- In the real use case I have here dozens of tables
)
SELECT
dealers.name dealer,
targets.vehicle,
targets.id
FROM
dealers
JOIN targets
ON dealers.name = targets.dealer_name
WHERE dealers.id in (54,12,456,315,468)

If the WHERE clause from the "bikes" subquery is removed then first
Index Scan on dealers_pk is made and then Bitmap Index Scans on
"bikes" and "cars" table using indexes on "dealer_name" columns.

---------------------------------------------------------------
-- Available also at: https://www.db-fiddle.com/f/cEXt8HXSaQzsQ2yBDA2Z4H/7
CREATE TABLE dealers AS
SELECT
id,
(SELECT string_agg(CHR(65+(random() * 25)::integer), '') FROM
generate_series(1, 4) WHERE id>0) name
FROM generate_series(1, 1000) AS id
;
ALTER TABLE dealers ADD primary key (id);
CREATE INDEX ON dealers(name);

CREATE TABLE bikes AS
SELECT
generate_series AS id,
(SELECT name FROM dealers WHERE dealers.id = (SELECT
(random()*1000)::int WHERE generate_series>0)) AS dealer_name,
(random()*12+50)::int as frame_size
FROM generate_series(1, 100000);
ALTER TABLE bikes ADD primary key (id);
CREATE INDEX ON bikes(dealer_name);

CREATE TABLE cars AS
SELECT
generate_series as id,
(SELECT name FROM dealers WHERE dealers.id = (SELECT
(random()*1000)::int WHERE generate_series>0)) AS dealer_name,
(random()*7+14)::int as wheel_size
FROM generate_series(1, 100000);
ALTER TABLE cars ADD primary key (id);
CREATE INDEX ON cars(dealer_name);

ANALYZE;

--
- Lauri