Improve Subquery Joins

#58 improved quite a few subquery joins, specifically SEMI JOINs, but others still don't push down consistently. An example using the taxi data from the tutorial:

create table trips_local as SELECT * FROM TRIPS limit 100;
explain (verbose)
SELECT *
 FROM (select vendor_id, count(*) num FROM trips group by vendor_id) AS agg
 JOIN trips_local l ON agg.vendor_id = l.vendor_id;

Produces this plan:

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     QUERY PLAN                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Merge Join  (cost=9.32..22.02 rows=100 width=287)
   Output: trips.vendor_id, (count(*)), l.trip_id, l.vendor_id, l.pickup_date, l.pickup_datetime, l.dropoff_date, l.dropoff_datetime, l.store_and_fwd_flag, l.rate_code_id, l.pickup_longitude, l.pickup_latitude, l.dropoff_longitude, l.dropoff_latitude, l.passenger_count, l.trip_distance, l.fare_amount, l.extra, l.mta_tax, l.tip_amount, l.tolls_amount, l.ehail_fee, l.improvement_surcharge, l.total_amount, l.payment_type, l.trip_type, l.pickup, l.dropoff, l.cab_type, l.pickup_nyct2010_gid, l.pickup_ctlabel, l.pickup_borocode, l.pickup_ct2010, l.pickup_boroct2010, l.pickup_cdeligibil, l.pickup_ntacode, l.pickup_ntaname, l.pickup_puma, l.dropoff_nyct2010_gid, l.dropoff_ctlabel, l.dropoff_borocode, l.dropoff_ct2010, l.dropoff_boroct2010, l.dropoff_cdeligibil, l.dropoff_ntacode, l.dropoff_ntaname, l.dropoff_puma
   Merge Cond: (trips.vendor_id = l.vendor_id)
   ->  GroupAggregate  (cost=1.00..12.50 rows=200 width=40)
         Output: trips.vendor_id, count(*)
         Group Key: trips.vendor_id
         ->  Foreign Scan on taxi.trips  (cost=1.00..5.50 rows=1000 width=32)
               Output: trips.trip_id, trips.vendor_id, trips.pickup_date, trips.pickup_datetime, trips.dropoff_date, trips.dropoff_datetime, trips.store_and_fwd_flag, trips.rate_code_id, trips.pickup_longitude, trips.pickup_latitude, trips.dropoff_longitude, trips.dropoff_latitude, trips.passenger_count, trips.trip_distance, trips.fare_amount, trips.extra, trips.mta_tax, trips.tip_amount, trips.tolls_amount, trips.ehail_fee, trips.improvement_surcharge, trips.total_amount, trips.payment_type, trips.trip_type, trips.pickup, trips.dropoff, trips.cab_type, trips.pickup_nyct2010_gid, trips.pickup_ctlabel, trips.pickup_borocode, trips.pickup_ct2010, trips.pickup_boroct2010, trips.pickup_cdeligibil, trips.pickup_ntacode, trips.pickup_ntaname, trips.pickup_puma, trips.dropoff_nyct2010_gid, trips.dropoff_ctlabel, trips.dropoff_borocode, trips.dropoff_ct2010, trips.dropoff_boroct2010, trips.dropoff_cdeligibil, trips.dropoff_ntacode, trips.dropoff_ntaname, trips.dropoff_puma
               Remote SQL: SELECT vendor_id FROM taxi.trips ORDER BY vendor_id ASC NULLS LAST
   ->  Sort  (cost=8.32..8.57 rows=100 width=247)
         Output: l.trip_id, l.vendor_id, l.pickup_date, l.pickup_datetime, l.dropoff_date, l.dropoff_datetime, l.store_and_fwd_flag, l.rate_code_id, l.pickup_longitude, l.pickup_latitude, l.dropoff_longitude, l.dropoff_latitude, l.passenger_count, l.trip_distance, l.fare_amount, l.extra, l.mta_tax, l.tip_amount, l.tolls_amount, l.ehail_fee, l.improvement_surcharge, l.total_amount, l.payment_type, l.trip_type, l.pickup, l.dropoff, l.cab_type, l.pickup_nyct2010_gid, l.pickup_ctlabel, l.pickup_borocode, l.pickup_ct2010, l.pickup_boroct2010, l.pickup_cdeligibil, l.pickup_ntacode, l.pickup_ntaname, l.pickup_puma, l.dropoff_nyct2010_gid, l.dropoff_ctlabel, l.dropoff_borocode, l.dropoff_ct2010, l.dropoff_boroct2010, l.dropoff_cdeligibil, l.dropoff_ntacode, l.dropoff_ntaname, l.dropoff_puma
         Sort Key: l.vendor_id
         ->  Seq Scan on taxi.trips_local l  (cost=0.00..5.00 rows=100 width=247)
               Output: l.trip_id, l.vendor_id, l.pickup_date, l.pickup_datetime, l.dropoff_date, l.dropoff_datetime, l.store_and_fwd_flag, l.rate_code_id, l.pickup_longitude, l.pickup_latitude, l.dropoff_longitude, l.dropoff_latitude, l.passenger_count, l.trip_distance, l.fare_amount, l.extra, l.mta_tax, l.tip_amount, l.tolls_amount, l.ehail_fee, l.improvement_surcharge, l.total_amount, l.payment_type, l.trip_type, l.pickup, l.dropoff, l.cab_type, l.pickup_nyct2010_gid, l.pickup_ctlabel, l.pickup_borocode, l.pickup_ct2010, l.pickup_boroct2010, l.pickup_cdeligibil, l.pickup_ntacode, l.pickup_ntaname, l.pickup_puma, l.dropoff_nyct2010_gid, l.dropoff_ctlabel, l.dropoff_borocode, l.dropoff_ct2010, l.dropoff_boroct2010, l.dropoff_cdeligibil, l.dropoff_ntacode, l.dropoff_ntaname, l.dropoff_puma
(14 rows)

Time: 33.099 ms

Which seems wild, frankly. Figure out whether it's possible to convince PostgreSQL to push down the list of l.vendor_id values to query directly against the trips table in ClickHouse.

Another

Another variant:

taxi=# explain (verbose) WITH agg AS (select vendor_id, count(*) num FROM trips group by vendor_id) select agg.vendor_id from agg;
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     QUERY PLAN                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Subquery Scan on agg  (cost=1.00..12.00 rows=200 width=32)
   Output: agg.vendor_id
   ->  GroupAggregate  (cost=1.00..10.00 rows=200 width=40)
         Output: trips.vendor_id, NULL::bigint
         Group Key: trips.vendor_id
         ->  Foreign Scan on taxi.trips  (cost=1.00..5.50 rows=1000 width=32)
               Output: trips.trip_id, trips.vendor_id, trips.pickup_date, trips.pickup_datetime, trips.dropoff_date, trips.dropoff_datetime, trips.store_and_fwd_flag, trips.rate_code_id, trips.pickup_longitude, trips.pickup_latitude, trips.dropoff_longitude, trips.dropoff_latitude, trips.passenger_count, trips.trip_distance, trips.fare_amount, trips.extra, trips.mta_tax, trips.tip_amount, trips.tolls_amount, trips.ehail_fee, trips.improvement_surcharge, trips.total_amount, trips.payment_type, trips.trip_type, trips.pickup, trips.dropoff, trips.cab_type, trips.pickup_nyct2010_gid, trips.pickup_ctlabel, trips.pickup_borocode, trips.pickup_ct2010, trips.pickup_boroct2010, trips.pickup_cdeligibil, trips.pickup_ntacode, trips.pickup_ntaname, trips.pickup_puma, trips.dropoff_nyct2010_gid, trips.dropoff_ctlabel, trips.dropoff_borocode, trips.dropoff_ct2010, trips.dropoff_boroct2010, trips.dropoff_cdeligibil, trips.dropoff_ntacode, trips.dropoff_ntaname, trips.dropoff_puma
               Remote SQL: SELECT vendor_id FROM taxi.trips ORDER BY vendor_id ASC NULLS LAST
(8 rows)

LIMIT Subquery

Here's another one:

taxi=# explain SELECT * FROM (select vendor_id, count(*) num FROM trips group by vendor_id) LIMIT 10;
                        QUERY PLAN                        
----------------------------------------------------------
 Limit  (cost=1.00..1.04 rows=10 width=40)
   ->  Foreign Scan  (cost=1.00..5.10 rows=1000 width=40)
         Relations: Aggregate on (trips)

Could LIMIT 10 be pushed down? Maybe it's enough to tell users to put the LIMIT in the subquery:

taxi=# explain SELECT * FROM (select vendor_id, count(*) num FROM trips group by vendor_id LIMIT 10);
                    QUERY PLAN                     
---------------------------------------------------
 Foreign Scan  (cost=0.00..-10.00 rows=1 width=40)
   Relations: Aggregate on (trips)