Columnar: always disable parallel paths. (#4617)

Previously, if columnar.enable_custom_scan was false, parallel paths
could remain, leading to an unexpected error.

Also, ensure that cheapest_parameterized_paths is cleared if a custom
scan is used.

Co-authored-by: Jeff Davis <jefdavi@microsoft.com>

src/backend/columnar/cstore_customscan.c

@@ -139,11 +139,12 @@ columnar_customscan_init()
 static void
 clear_paths(RelOptInfo *rel)
 {
-	rel->pathlist = NULL;
-	rel->partial_pathlist = NULL;
+	rel->pathlist = NIL;
+	rel->partial_pathlist = NIL;
 	rel->cheapest_startup_path = NULL;
 	rel->cheapest_total_path = NULL;
 	rel->cheapest_unique_path = NULL;
+	rel->cheapest_parameterized_paths = NIL;
 }
 
 
@@ -157,12 +158,6 @@ ColumnarSetRelPathlistHook(PlannerInfo *root, RelOptInfo *rel, Index rti,
 		PreviousSetRelPathlistHook(root, rel, rti, rte);
 	}
 
-	if (!EnableColumnarCustomScan)
-	{
-		/* custon scans are disabled, use normal table access method api instead */
-		return;
-	}
-
 	if (!OidIsValid(rte->relid) || rte->rtekind != RTE_RELATION)
 	{
 		/* some calls to the pathlist hook don't have a valid relation set. Do nothing */
@@ -183,13 +178,19 @@ ColumnarSetRelPathlistHook(PlannerInfo *root, RelOptInfo *rel, Index rti,
 							errmsg("sample scans not supported on columnar tables")));
 		}
 
-		Path *customPath = CreateColumnarScanPath(root, rel, rte);
+		/* columnar doesn't support parallel paths */
+		rel->partial_pathlist = NIL;
 
-		ereport(DEBUG1, (errmsg("pathlist hook for columnar table am")));
+		if (EnableColumnarCustomScan)
+		{
+			Path *customPath = CreateColumnarScanPath(root, rel, rte);
 
-		/* we propose a new path that will be the only path for scanning this relation */
-		clear_paths(rel);
-		add_path(rel, customPath);
+			ereport(DEBUG1, (errmsg("pathlist hook for columnar table am")));
+
+			/* we propose a new path that will be the only path for scanning this relation */
+			clear_paths(rel);
+			add_path(rel, customPath);
+		}
 	}
 	RelationClose(relation);
 }

src/test/regress/columnar_am_schedule

@@ -8,6 +8,7 @@ test: am_query
 test: am_analyze
 test: am_data_types
 test: am_drop
+test: columnar_fallback_scan
 test: am_empty
 test: am_insert
 test: am_update_delete

src/test/regress/expected/columnar_fallback_scan.out

@@ -0,0 +1,53 @@
+--
+-- columnar_fallback_scan.sql
+--
+-- Test columnar.enable_custom_scan = false, which will use an
+-- ordinary sequential scan. It won't benefit from projection pushdown
+-- or qual pushdown, but it should return the correct results.
+--
+set columnar.enable_custom_scan = false;
+create table fallback_scan(i int) using columnar;
+-- large enough to test parallel_workers > 1
+select alter_columnar_table_set('fallback_scan', compression => 'none');
+ alter_columnar_table_set
+---------------------------------------------------------------------
+
+(1 row)
+
+insert into fallback_scan select generate_series(1,150000);
+vacuum analyze fallback_scan;
+select count(*), min(i), max(i), avg(i) from fallback_scan;
+ count  | min |  max   |        avg
+---------------------------------------------------------------------
+ 150000 |   1 | 150000 | 75000.500000000000
+(1 row)
+
+--
+-- Negative test: try to force a parallel plan with at least two
+-- workers, but columnar should reject it and use a non-parallel scan.
+--
+set force_parallel_mode = regress;
+set min_parallel_table_scan_size = 1;
+set parallel_tuple_cost = 0;
+set max_parallel_workers = 4;
+set max_parallel_workers_per_gather = 4;
+explain (costs off) select count(*), min(i), max(i), avg(i) from fallback_scan;
+           QUERY PLAN
+---------------------------------------------------------------------
+ Aggregate
+   ->  Seq Scan on fallback_scan
+(2 rows)
+
+select count(*), min(i), max(i), avg(i) from fallback_scan;
+ count  | min |  max   |        avg
+---------------------------------------------------------------------
+ 150000 |   1 | 150000 | 75000.500000000000
+(1 row)
+
+set force_parallel_mode to default;
+set min_parallel_table_scan_size to default;
+set parallel_tuple_cost to default;
+set max_parallel_workers to default;
+set max_parallel_workers_per_gather to default;
+set columnar.enable_custom_scan to default;
+drop table fallback_scan;

src/test/regress/sql/columnar_fallback_scan.sql

@@ -0,0 +1,39 @@
+--
+-- columnar_fallback_scan.sql
+--
+-- Test columnar.enable_custom_scan = false, which will use an
+-- ordinary sequential scan. It won't benefit from projection pushdown
+-- or qual pushdown, but it should return the correct results.
+--
+
+set columnar.enable_custom_scan = false;
+
+create table fallback_scan(i int) using columnar;
+-- large enough to test parallel_workers > 1
+select alter_columnar_table_set('fallback_scan', compression => 'none');
+insert into fallback_scan select generate_series(1,150000);
+vacuum analyze fallback_scan;
+
+select count(*), min(i), max(i), avg(i) from fallback_scan;
+
+--
+-- Negative test: try to force a parallel plan with at least two
+-- workers, but columnar should reject it and use a non-parallel scan.
+--
+set force_parallel_mode = regress;
+set min_parallel_table_scan_size = 1;
+set parallel_tuple_cost = 0;
+set max_parallel_workers = 4;
+set max_parallel_workers_per_gather = 4;
+explain (costs off) select count(*), min(i), max(i), avg(i) from fallback_scan;
+select count(*), min(i), max(i), avg(i) from fallback_scan;
+
+set force_parallel_mode to default;
+set min_parallel_table_scan_size to default;
+set parallel_tuple_cost to default;
+set max_parallel_workers to default;
+set max_parallel_workers_per_gather to default;
+
+set columnar.enable_custom_scan to default;
+
+drop table fallback_scan;