Question about optimising (Postgres_)FDW

Hi

I am playing around with postgres_fdw and found that the following code ...

----------------------------------
CREATE EXTENSION postgres_fdw;

CREATE SERVER loop foreign data wrapper postgres_fdw
OPTIONS (port '5432', dbname 'testdb');

CREATE USER MAPPING FOR PUBLIC SERVER loop;

create table onemillion (
id serial primary key,
inserted timestamp default clock_timestamp(),
data text
);

insert into onemillion(data) select random() from
generate_series(1,1000000);

CREATE FOREIGN TABLE onemillion_pgfdw (
id int,
inserted timestamp,
data text
) SERVER loop
OPTIONS (table_name 'onemillion',
use_remote_estimate 'true');

testdb=# explain analyse
select * from onemillion_pgfdw where id in (select id from onemillion
where data > '0.9' limit 100);
QUERY
PLAN
---------------------------------------------------------------------------------------------------------------------------------
Nested Loop (cost=122.49..10871.06 rows=500000 width=44) (actual
time=4.269..93.444 rows=100 loops=1)
-> HashAggregate (cost=22.06..23.06 rows=100 width=4) (actual
time=1.110..1.263 rows=100 loops=1)
-> Limit (cost=0.00..20.81 rows=100 width=4) (actual
time=0.038..1.026 rows=100 loops=1)
-> Seq Scan on onemillion (cost=0.00..20834.00
rows=100115 width=4) (actual time=0.036..0.984 rows=100 loops=1)
Filter: (data > '0.9'::text)
Rows Removed by Filter: 805
-> Foreign Scan on onemillion_pgfdw (cost=100.43..108.47 rows=1
width=29) (actual time=0.772..0.773 rows=1 loops=100)
Total runtime: 93.820 ms
(8 rows)

Time: 97.283 ms
------------------------------

... actually performs 100 distinct "SELECT * FROM onemillion WHERE id =
$1" calls on "remote" side.

Is there a way to force it to prefer a plan where the results of (select
id from onemillion where data > '0.9' limit 100)
are passed to FDW as a single IN ( = ANY(...)) query and are retrieved
all at once ?

If not, how hord would it be to add this feature ?

--
Hannu Krosing
PostgreSQL Consultant
Performance, Scalability and High Availability
2ndQuadrant Nordic OÜ