Group and aggregate functions seems to not be mapped and forwarded.

[thebf]

Test-query:
select fr_item,sum(quantity),transaction_week from transaction_treated where transaction_year = 2019 group by fr_item,transaction_week

Running query directly in Clickhouse returns the expected result with this summary:
3265 rows in set. Elapsed: 0.060 sec. Processed 411.75 thousand rows, 6.59 MB (6.90 million rows/s., 110.47 MB/s.)

Running the same query against the foreign table in PG returns the following plan and resulting execution time:

QUERY PLAN
GroupAggregate (cost=20.00..1530749.21 rows=1 width=12) (actual time=133.919..14199.340 rows=3265 loops=1)
Group Key: fr_item, transaction_week
-> Foreign Scan on transaction_treated (cost=20.00..1530032.00 rows=95627 width=12) (actual time=133.779..14128.219 rows=95627 loops=1)
Filter: (transaction_year = 2019)
Multicorn: SELECT transaction_week, transaction_year, fr_item, quantity
Multicorn: FROM transaction_treated
Multicorn: WHERE transaction_year = 2019
Multicorn: ORDER BY fr_item, transaction_week
Planning time: 13.510 ms
Execution time: 14200.668 ms

Being able to forward group by and simple aggregates like count and sum would increase the general value of this component in a lot of use-cases.