BUG #15947: Worse plan is chosen after giving the planner more freedom (partitionwise join)

The following bug has been logged on the website:

Bug reference: 15947
Logged by: Yaroslav Schekin
Email address: ladayaroslav(at)yandex(dot)ru
PostgreSQL version: 11.5
Operating system: Any
Description:

After creating of the tables below:
-----
CREATE TABLE sg (
id bigint NOT NULL,
sc_fk bigint,
geo_id bigint,
sl smallint NOT NULL,
a date NOT NULL,
o boolean NOT NULL
)
PARTITION BY RANGE (o, sl, a);

CREATE TABLE sg_19_01_d PARTITION OF sg FOR VALUES FROM (false, '5',
'2019-01-01') TO (false, '5', '2019-02-01');
CREATE TABLE sg_19_02_d PARTITION OF sg FOR VALUES FROM (false, '5',
'2019-02-01') TO (false, '5', '2019-03-01');

CREATE TABLE sc (
id bigint,
a date NOT NULL,
sl smallint NOT NULL,
o boolean NOT NULL
)
PARTITION BY RANGE (o, sl, a);

CREATE TABLE sc_19_01_d PARTITION OF sc FOR VALUES FROM (false, '5',
'2019-01-01') TO (false, '5', '2019-02-01');
CREATE TABLE sc_19_02_d PARTITION OF sc FOR VALUES FROM (false, '5',
'2019-02-01') TO (false, '5', '2019-03-01');

INSERT INTO sg_19_01_d(id, sc_fk, geo_id, sl, a, o)
SELECT n, n, 0, 5, '2019-01-01', false
FROM generate_series(1, 1000) AS g(n);

INSERT INTO sg_19_02_d(id, sc_fk, geo_id, sl, a, o)
SELECT n, n, 0, 5, '2019-02-01', false
FROM generate_series(1, 1000) AS g(n);

INSERT INTO sc_19_01_d(id, a, sl, o)
SELECT n, '2019-01-01', 5, false
FROM generate_series(1, 1000) AS g(n);

INSERT INTO sc_19_02_d(id, a, sl, o)
SELECT n, '2019-02-01', 5, false
FROM generate_series(1, 1000) AS g(n);

ANALYZE sg_19_01_d, sg_19_02_d, sc_19_01_d, sc_19_02_d;
-----
I'm trying the following query:

EXPLAIN
SELECT COUNT(*)
FROM sc
WHERE EXISTS (
SELECT 1
FROM sg
WHERE sc.id = sg.sc_fk
AND sc.a = sg.a
AND sc.o = sg.o
AND sc.sl = sg.sl
);

Which produces the plan with this cost estimation (top node):
-- Aggregate (cost=147.25..147.26 rows=1 width=8)

But after:
SET enable_partitionwise_join = true;

The new plan is more expensive:
-- Aggregate (cost=175.00..175.01 rows=1 width=8)

This shouldn't be happening, right?