Fix a crash involving a view on a hypertable

.unreleased/bugfix_6796

@@ -0,0 +1 @@
+Fixes: #6796 Fix a crash involving a view on a hypertable

src/nodes/chunk_append/chunk_append.c

@@ -348,9 +348,6 @@ ts_chunk_append_path_create(PlannerInfo *root, RelOptInfo *rel, Hypertable *ht,
 			List *merge_childs = NIL;
 			MergeAppendPath *append;
 
-			if (flat == NULL)
-				break;
-
 			/*
 			 * For each lc_oid, there will be 0, 1, or 2 matches in flat_list: 0 matches
 			 * if child was pruned, 1 match if the chunk is uncompressed or fully compressed,
@@ -363,6 +360,10 @@ ts_chunk_append_path_create(PlannerInfo *root, RelOptInfo *rel, Hypertable *ht,
 #ifdef USE_ASSERT_CHECKING
 				int nmatches = 0;
 #endif
+				/* Before entering the "DO" loop, check for a valid path entry */
+				if (flat == NULL)
+					break;
+
 				do
 				{
 					Path *child = (Path *) lfirst(flat);

test/expected/chunk_utils.out

@@ -1556,3 +1556,48 @@ SELECT drop_chunks('drop_chunk_test_ts', created_after => INTERVAL '1 hour', cre
  _timescaledb_internal._hyper_4_20_chunk
 (2 rows)
 
+-- Test views on top of hypertables
+CREATE TABLE view_test (project_id INT, ts TIMESTAMPTZ NOT NULL);
+SELECT create_hypertable('view_test', by_range('ts', INTERVAL '1 day'));
+ create_hypertable 
+-------------------
+ (17,t)
+(1 row)
+
+-- exactly one partition per project_id
+SELECT * FROM add_dimension('view_test', 'project_id', chunk_time_interval => 1); -- exactly one partition per project; works for *integer* types
+NOTICE:  adding not-null constraint to column "project_id"
+ dimension_id | schema_name | table_name | column_name | created 
+--------------+-------------+------------+-------------+---------
+           22 | try_schema  | view_test  | project_id  | t
+(1 row)
+
+INSERT INTO view_test (project_id, ts)
+SELECT g % 25 + 1 AS project_id, i.ts + (g * interval '1 week') / i.total AS ts
+FROM (SELECT timestamptz '2024-01-01 00:00:00+0', 600) i(ts, total),
+generate_series(1, i.total) g;
+-- Create a view on top of this hypertable
+CREATE VIEW test_view_part_few AS SELECT project_id,
+    ts
+   FROM view_test
+  WHERE project_id = ANY (ARRAY[5, 10, 15]);
+-- Complicated query on a view involving a range check and a sort
+SELECT * FROM test_view_part_few WHERE ts BETWEEN '2024-01-04 00:00:00+00'AND '2024-01-05 00:00:00' ORDER BY ts LIMIT 1000;
+ project_id |              ts              
+------------+------------------------------
+         10 | Wed Jan 03 16:31:12 2024 PST
+         15 | Wed Jan 03 17:55:12 2024 PST
+          5 | Wed Jan 03 22:07:12 2024 PST
+         10 | Wed Jan 03 23:31:12 2024 PST
+         15 | Thu Jan 04 00:55:12 2024 PST
+          5 | Thu Jan 04 05:07:12 2024 PST
+         10 | Thu Jan 04 06:31:12 2024 PST
+         15 | Thu Jan 04 07:55:12 2024 PST
+          5 | Thu Jan 04 12:07:12 2024 PST
+         10 | Thu Jan 04 13:31:12 2024 PST
+         15 | Thu Jan 04 14:55:12 2024 PST
+          5 | Thu Jan 04 19:07:12 2024 PST
+         10 | Thu Jan 04 20:31:12 2024 PST
+         15 | Thu Jan 04 21:55:12 2024 PST
+(14 rows)
+

test/sql/chunk_utils.sql

@@ -655,3 +655,20 @@ SELECT show_chunks('drop_chunk_test_ts');
 -- timestamp
 SELECT show_chunks('drop_chunk_test_ts', created_after => now() - INTERVAL '1 hour', created_before => now());
 SELECT drop_chunks('drop_chunk_test_ts', created_after => INTERVAL '1 hour', created_before => now());
+
+-- Test views on top of hypertables
+CREATE TABLE view_test (project_id INT, ts TIMESTAMPTZ NOT NULL);
+SELECT create_hypertable('view_test', by_range('ts', INTERVAL '1 day'));
+-- exactly one partition per project_id
+SELECT * FROM add_dimension('view_test', 'project_id', chunk_time_interval => 1); -- exactly one partition per project; works for *integer* types
+INSERT INTO view_test (project_id, ts)
+SELECT g % 25 + 1 AS project_id, i.ts + (g * interval '1 week') / i.total AS ts
+FROM (SELECT timestamptz '2024-01-01 00:00:00+0', 600) i(ts, total),
+generate_series(1, i.total) g;
+-- Create a view on top of this hypertable
+CREATE VIEW test_view_part_few AS SELECT project_id,
+    ts
+   FROM view_test
+  WHERE project_id = ANY (ARRAY[5, 10, 15]);
+-- Complicated query on a view involving a range check and a sort
+SELECT * FROM test_view_part_few WHERE ts BETWEEN '2024-01-04 00:00:00+00'AND '2024-01-05 00:00:00' ORDER BY ts LIMIT 1000;