BUG #15383: Join Filter cost estimation problem in 10.5

The following bug has been logged on the website:

Bug reference:      15383
Logged by:          Marko Tiikkaja
Email address:      marko@joh.to
PostgreSQL version: 10.5
Operating system:   Linux
Description:

I was looking at a problematic plan submitted by "sjamaan" on IRC and I
noticed that the join filter estimation seems completely wrong here:

create function expensive_func(int) returns int as $$ begin return 1; end $$
language plpgsql stable cost 10000;
create table unique_inner(a int primary key);
insert into unique_inner select generate_series(1, 10000);

explain select * from unique_inner gs1(i) join generate_series(1, 10) gs2(i)
using (i) where expensive_func(gs1.i + gs2.i) > 0;
                                    QUERY PLAN
----------------------------------------------------------------------------------
 Hash Join  (cost=303.19..315.81 rows=333 width=4)
   Hash Cond: (gs2.i = gs1.i)
   Join Filter: (expensive_func((gs1.i + gs2.i)) > 0)
   ->  Function Scan on generate_series gs2  (cost=0.00..10.00 rows=1000
width=4)
   ->  Hash  (cost=159.75..159.75 rows=11475 width=4)
         ->  Seq Scan on unique_inner gs1  (cost=0.00..159.75 rows=11475
width=4)
(6 rows)

(Notice how even though the function is expected to be called at least 333
times, the cost doesn't account for even a single call.)

Dropping the primary key constraint makes the costs more reasonable (though
I'm still not sure how the planner arrives at these costs):

alter table unique_inner drop constraint unique_inner_pkey;
explain select * from unique_inner gs1(i) join generate_series(1, 10) gs2(i)
using (i) where expensive_func(gs1.i + gs2.i) > 0;
                                       QUERY PLAN
----------------------------------------------------------------------------------------
 Hash Join  (cost=22.50..1436880.94 rows=19125 width=4)
   Hash Cond: (gs1.i = gs2.i)
   Join Filter: (expensive_func((gs1.i + gs2.i)) > 0)
   ->  Seq Scan on unique_inner gs1  (cost=0.00..159.75 rows=11475
width=4)
   ->  Hash  (cost=10.00..10.00 rows=1000 width=4)
         ->  Function Scan on generate_series gs2  (cost=0.00..10.00
rows=1000 width=4)
(6 rows)