Avoid generating bogus paths with partitionwise aggregate.

Previously, if some or all partitions had no partially aggregated path,
we would still try to generate a partially aggregated path for the
parent, leading to assertion failures or wrong answers.

Report by Rajkumar Raghuwanshi.  Patch by Jeevan Chalke, reviewed
by Ashutosh Bapat.  A few changes by me.

Discussion: http://postgr.es/m/CAKcux6=q4+Mw8gOOX16ef6ZMFp9Cve7KWFstUsrDa4GiFaXGUQ@mail.gmail.com

src/test/regress/expected/partition_aggregate.out

@@ -1394,3 +1394,108 @@ SELECT y, sum(x), avg(x), count(*) FROM pagg_tab_para GROUP BY y HAVING avg(x) <
  11 | 16500 | 11.0000000000000000 |  1500
 (4 rows)
 
+-- Test when parent can produce parallel paths but not any (or some) of its children
+ALTER TABLE pagg_tab_para_p1 SET (parallel_workers = 0);
+ALTER TABLE pagg_tab_para_p3 SET (parallel_workers = 0);
+ANALYZE pagg_tab_para;
+EXPLAIN (COSTS OFF)
+SELECT x, sum(y), avg(y), count(*) FROM pagg_tab_para GROUP BY x HAVING avg(y) < 7 ORDER BY 1, 2, 3;
+                                      QUERY PLAN                                      
+--------------------------------------------------------------------------------------
+ Sort
+   Sort Key: pagg_tab_para_p1.x, (sum(pagg_tab_para_p1.y)), (avg(pagg_tab_para_p1.y))
+   ->  Finalize GroupAggregate
+         Group Key: pagg_tab_para_p1.x
+         Filter: (avg(pagg_tab_para_p1.y) < '7'::numeric)
+         ->  Gather Merge
+               Workers Planned: 2
+               ->  Sort
+                     Sort Key: pagg_tab_para_p1.x
+                     ->  Partial HashAggregate
+                           Group Key: pagg_tab_para_p1.x
+                           ->  Parallel Append
+                                 ->  Seq Scan on pagg_tab_para_p1
+                                 ->  Seq Scan on pagg_tab_para_p3
+                                 ->  Parallel Seq Scan on pagg_tab_para_p2
+(15 rows)
+
+SELECT x, sum(y), avg(y), count(*) FROM pagg_tab_para GROUP BY x HAVING avg(y) < 7 ORDER BY 1, 2, 3;
+ x  | sum  |        avg         | count 
+----+------+--------------------+-------
+  0 | 5000 | 5.0000000000000000 |  1000
+  1 | 6000 | 6.0000000000000000 |  1000
+ 10 | 5000 | 5.0000000000000000 |  1000
+ 11 | 6000 | 6.0000000000000000 |  1000
+ 20 | 5000 | 5.0000000000000000 |  1000
+ 21 | 6000 | 6.0000000000000000 |  1000
+(6 rows)
+
+ALTER TABLE pagg_tab_para_p2 SET (parallel_workers = 0);
+ANALYZE pagg_tab_para;
+EXPLAIN (COSTS OFF)
+SELECT x, sum(y), avg(y), count(*) FROM pagg_tab_para GROUP BY x HAVING avg(y) < 7 ORDER BY 1, 2, 3;
+                                      QUERY PLAN                                      
+--------------------------------------------------------------------------------------
+ Sort
+   Sort Key: pagg_tab_para_p1.x, (sum(pagg_tab_para_p1.y)), (avg(pagg_tab_para_p1.y))
+   ->  Finalize GroupAggregate
+         Group Key: pagg_tab_para_p1.x
+         Filter: (avg(pagg_tab_para_p1.y) < '7'::numeric)
+         ->  Gather Merge
+               Workers Planned: 2
+               ->  Sort
+                     Sort Key: pagg_tab_para_p1.x
+                     ->  Partial HashAggregate
+                           Group Key: pagg_tab_para_p1.x
+                           ->  Parallel Append
+                                 ->  Seq Scan on pagg_tab_para_p1
+                                 ->  Seq Scan on pagg_tab_para_p2
+                                 ->  Seq Scan on pagg_tab_para_p3
+(15 rows)
+
+SELECT x, sum(y), avg(y), count(*) FROM pagg_tab_para GROUP BY x HAVING avg(y) < 7 ORDER BY 1, 2, 3;
+ x  | sum  |        avg         | count 
+----+------+--------------------+-------
+  0 | 5000 | 5.0000000000000000 |  1000
+  1 | 6000 | 6.0000000000000000 |  1000
+ 10 | 5000 | 5.0000000000000000 |  1000
+ 11 | 6000 | 6.0000000000000000 |  1000
+ 20 | 5000 | 5.0000000000000000 |  1000
+ 21 | 6000 | 6.0000000000000000 |  1000
+(6 rows)
+
+-- Reset parallelism parameters to get partitionwise aggregation plan.
+RESET min_parallel_table_scan_size;
+RESET parallel_setup_cost;
+EXPLAIN (COSTS OFF)
+SELECT x, sum(y), avg(y), count(*) FROM pagg_tab_para GROUP BY x HAVING avg(y) < 7 ORDER BY 1, 2, 3;
+                                      QUERY PLAN                                      
+--------------------------------------------------------------------------------------
+ Sort
+   Sort Key: pagg_tab_para_p1.x, (sum(pagg_tab_para_p1.y)), (avg(pagg_tab_para_p1.y))
+   ->  Append
+         ->  HashAggregate
+               Group Key: pagg_tab_para_p1.x
+               Filter: (avg(pagg_tab_para_p1.y) < '7'::numeric)
+               ->  Seq Scan on pagg_tab_para_p1
+         ->  HashAggregate
+               Group Key: pagg_tab_para_p2.x
+               Filter: (avg(pagg_tab_para_p2.y) < '7'::numeric)
+               ->  Seq Scan on pagg_tab_para_p2
+         ->  HashAggregate
+               Group Key: pagg_tab_para_p3.x
+               Filter: (avg(pagg_tab_para_p3.y) < '7'::numeric)
+               ->  Seq Scan on pagg_tab_para_p3
+(15 rows)
+
+SELECT x, sum(y), avg(y), count(*) FROM pagg_tab_para GROUP BY x HAVING avg(y) < 7 ORDER BY 1, 2, 3;
+ x  | sum  |        avg         | count 
+----+------+--------------------+-------
+  0 | 5000 | 5.0000000000000000 |  1000
+  1 | 6000 | 6.0000000000000000 |  1000
+ 10 | 5000 | 5.0000000000000000 |  1000
+ 11 | 6000 | 6.0000000000000000 |  1000
+ 20 | 5000 | 5.0000000000000000 |  1000
+ 21 | 6000 | 6.0000000000000000 |  1000
+(6 rows)
+

src/test/regress/sql/partition_aggregate.sql

@@ -294,3 +294,27 @@ SELECT x, sum(y), avg(y), count(*) FROM pagg_tab_para GROUP BY x HAVING avg(y) <
 EXPLAIN (COSTS OFF)
 SELECT y, sum(x), avg(x), count(*) FROM pagg_tab_para GROUP BY y HAVING avg(x) < 12 ORDER BY 1, 2, 3;
 SELECT y, sum(x), avg(x), count(*) FROM pagg_tab_para GROUP BY y HAVING avg(x) < 12 ORDER BY 1, 2, 3;
+
+-- Test when parent can produce parallel paths but not any (or some) of its children
+ALTER TABLE pagg_tab_para_p1 SET (parallel_workers = 0);
+ALTER TABLE pagg_tab_para_p3 SET (parallel_workers = 0);
+ANALYZE pagg_tab_para;
+
+EXPLAIN (COSTS OFF)
+SELECT x, sum(y), avg(y), count(*) FROM pagg_tab_para GROUP BY x HAVING avg(y) < 7 ORDER BY 1, 2, 3;
+SELECT x, sum(y), avg(y), count(*) FROM pagg_tab_para GROUP BY x HAVING avg(y) < 7 ORDER BY 1, 2, 3;
+
+ALTER TABLE pagg_tab_para_p2 SET (parallel_workers = 0);
+ANALYZE pagg_tab_para;
+
+EXPLAIN (COSTS OFF)
+SELECT x, sum(y), avg(y), count(*) FROM pagg_tab_para GROUP BY x HAVING avg(y) < 7 ORDER BY 1, 2, 3;
+SELECT x, sum(y), avg(y), count(*) FROM pagg_tab_para GROUP BY x HAVING avg(y) < 7 ORDER BY 1, 2, 3;
+
+-- Reset parallelism parameters to get partitionwise aggregation plan.
+RESET min_parallel_table_scan_size;
+RESET parallel_setup_cost;
+
+EXPLAIN (COSTS OFF)
+SELECT x, sum(y), avg(y), count(*) FROM pagg_tab_para GROUP BY x HAVING avg(y) < 7 ORDER BY 1, 2, 3;
+SELECT x, sum(y), avg(y), count(*) FROM pagg_tab_para GROUP BY x HAVING avg(y) < 7 ORDER BY 1, 2, 3;