postgres_fdw: Push down partition-wise aggregation.

Since commit 7012b132d0, postgres_fdw
has been able to push down the toplevel aggregation operation to the
remote server.  Commit e2f1eb0ee3 made
it possible to break down the toplevel aggregation into one
aggregate per partition.  This commit lets postgres_fdw push down
aggregation in that case just as it does at the top level.

In order to make this work, this commit adds an additional argument
to the GetForeignUpperPaths FDW API.  A matching argument is added
to the signature for create_upper_paths_hook.  Third-party code using
either of these will need to be updated.

Also adjust create_foreignscan_plan() so that it picks up the correct
set of relids in this case.

Jeevan Chalke, reviewed by Ashutosh Bapat and by me and with some
adjustments by me.  The larger patch series of which this patch is a
part was also reviewed and tested by Antonin Houska, Rajkumar
Raghuwanshi, David Rowley, Dilip Kumar, Konstantin Knizhnik, Pascal
Legrand, and Rafia Sabih.

Discussion: http://postgr.es/m/CAM2+6=V64_xhstVHie0Rz=KPEQnLJMZt_e314P0jaT_oJ9MR8A@mail.gmail.com
Discussion: http://postgr.es/m/CAM2+6=XPWujjmj5zUaBTGDoB38CemwcPmjkRy0qOcsQj_V+2sQ@mail.gmail.com

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -1932,3 +1932,54 @@ SELECT t1.a, t1.phv, t2.b, t2.phv FROM (SELECT 't1_phv' phv, * FROM fprt1 WHERE
 SELECT t1.a, t1.phv, t2.b, t2.phv FROM (SELECT 't1_phv' phv, * FROM fprt1 WHERE a % 25 = 0) t1 FULL JOIN (SELECT 't2_phv' phv, * FROM fprt2 WHERE b % 25 = 0) t2 ON (t1.a = t2.b) ORDER BY t1.a, t2.b;
 
 RESET enable_partitionwise_join;
+
+
+-- ===================================================================
+-- test partitionwise aggregates
+-- ===================================================================
+
+CREATE TABLE pagg_tab (a int, b int, c text) PARTITION BY RANGE(a);
+
+CREATE TABLE pagg_tab_p1 (LIKE pagg_tab);
+CREATE TABLE pagg_tab_p2 (LIKE pagg_tab);
+CREATE TABLE pagg_tab_p3 (LIKE pagg_tab);
+
+INSERT INTO pagg_tab_p1 SELECT i % 30, i % 50, to_char(i/30, 'FM0000') FROM generate_series(1, 3000) i WHERE (i % 30) < 10;
+INSERT INTO pagg_tab_p2 SELECT i % 30, i % 50, to_char(i/30, 'FM0000') FROM generate_series(1, 3000) i WHERE (i % 30) < 20 and (i % 30) >= 10;
+INSERT INTO pagg_tab_p3 SELECT i % 30, i % 50, to_char(i/30, 'FM0000') FROM generate_series(1, 3000) i WHERE (i % 30) < 30 and (i % 30) >= 20;
+
+-- Create foreign partitions
+CREATE FOREIGN TABLE fpagg_tab_p1 PARTITION OF pagg_tab FOR VALUES FROM (0) TO (10) SERVER loopback OPTIONS (table_name 'pagg_tab_p1');
+CREATE FOREIGN TABLE fpagg_tab_p2 PARTITION OF pagg_tab FOR VALUES FROM (10) TO (20) SERVER loopback OPTIONS (table_name 'pagg_tab_p2');;
+CREATE FOREIGN TABLE fpagg_tab_p3 PARTITION OF pagg_tab FOR VALUES FROM (20) TO (30) SERVER loopback OPTIONS (table_name 'pagg_tab_p3');;
+
+ANALYZE pagg_tab;
+ANALYZE fpagg_tab_p1;
+ANALYZE fpagg_tab_p2;
+ANALYZE fpagg_tab_p3;
+
+-- When GROUP BY clause matches with PARTITION KEY.
+-- Plan with partitionwise aggregates is disabled
+SET enable_partitionwise_aggregate TO false;
+EXPLAIN (COSTS OFF)
+SELECT a, sum(b), min(b), count(*) FROM pagg_tab GROUP BY a HAVING avg(b) < 22 ORDER BY 1;
+
+-- Plan with partitionwise aggregates is enabled
+SET enable_partitionwise_aggregate TO true;
+EXPLAIN (COSTS OFF)
+SELECT a, sum(b), min(b), count(*) FROM pagg_tab GROUP BY a HAVING avg(b) < 22 ORDER BY 1;
+SELECT a, sum(b), min(b), count(*) FROM pagg_tab GROUP BY a HAVING avg(b) < 22 ORDER BY 1;
+
+-- Check with whole-row reference
+-- Should have all the columns in the target list for the given relation
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT a, count(t1) FROM pagg_tab t1 GROUP BY a HAVING avg(b) < 22 ORDER BY 1;
+SELECT a, count(t1) FROM pagg_tab t1 GROUP BY a HAVING avg(b) < 22 ORDER BY 1;
+
+-- When GROUP BY clause does not match with PARTITION KEY.
+EXPLAIN (COSTS OFF)
+SELECT b, avg(a), max(a), count(*) FROM pagg_tab GROUP BY b HAVING sum(a) < 700 ORDER BY 1;
+
+
+-- Clean-up
+RESET enable_partitionwise_aggregate;