fix citusdata#7647 Add comment and test

src/backend/columnar/columnar_customscan.c

@@ -1052,6 +1052,10 @@ FindCandidateRelids(PlannerInfo *root, RelOptInfo *rel, List *joinClauses)
 	candidateRelids = bms_del_members(candidateRelids, rel->relids);
 	candidateRelids = bms_del_members(candidateRelids, rel->lateral_relids);
 
+	/*
+	 * For the relevant PG16 commit requiring this addition:
+	 * postgres/postgres@2489d76
+	 */
 #if PG_VERSION_NUM >= PG_VERSION_16
 	candidateRelids = bms_del_members(candidateRelids, root->outer_join_rels);
 #endif

src/test/regress/expected/columnar_join.out

@@ -356,6 +356,33 @@ ORDER BY 3,4,1,2;
        3 |       30 |           6 |           60
 (9 rows)
 
+-- Left Join with Mixed Table Types and columnar in the middle
+CREATE TABLE tbl_middle_left_heap1 (id integer);
+CREATE TABLE tbl_middle_left_heap2 (id integer);
+CREATE TABLE tbl_middle_left_columnar (id integer) USING columnar;
+INSERT INTO tbl_middle_left_heap1 VALUES (1), (2), (3), (4);
+INSERT INTO tbl_middle_left_heap2 VALUES (2), (3), (5), (6);
+INSERT INTO tbl_middle_left_columnar VALUES (3), (5), (7);
+EXPLAIN (COSTS OFF)
+SELECT h1.*, h2.*, c.*
+FROM tbl_middle_left_heap1 h1
+LEFT JOIN tbl_middle_left_columnar c ON h1.id = c.id
+LEFT JOIN tbl_middle_left_heap2 h2 ON c.id = h2.id
+ORDER BY 1;
+                        QUERY PLAN
+---------------------------------------------------------------------
+Sort
+  Sort Key: h1.id
+  ->  Nested Loop Left Join
+        Join Filter: (c.id = h2.id)
+        ->  Nested Loop Left Join
+              Join Filter: (h1.id = c.id)
+              ->  Seq Scan on tbl_middle_left_heap1 h1
+              ->  Custom Scan (ColumnarScan) on tbl_middle_left_columnar c
+                    Columnar Projected Columns: id
+        ->  Seq Scan on tbl_middle_left_heap2 h2
+(10 rows)
+
 -- End test case
 SET client_min_messages TO warning;
 DROP SCHEMA am_columnar_join CASCADE;

src/test/regress/sql/columnar_join.sql

@@ -163,6 +163,22 @@ FROM tbl_cross_heap h
 CROSS JOIN tbl_cross_columnar c
 ORDER BY 3,4,1,2;
 
+-- Left Join with Mixed Table Types and columnar in the middle
+CREATE TABLE tbl_middle_left_heap1 (id integer);
+CREATE TABLE tbl_middle_left_heap2 (id integer);
+CREATE TABLE tbl_middle_left_columnar (id integer) USING columnar;
+
+INSERT INTO tbl_middle_left_heap1 VALUES (1), (2), (3), (4);
+INSERT INTO tbl_middle_left_heap2 VALUES (2), (3), (5), (6);
+INSERT INTO tbl_middle_left_columnar VALUES (3), (5), (7);
+
+EXPLAIN (COSTS OFF)
+SELECT h1.*, h2.*, c.*
+FROM tbl_middle_left_heap1 h1
+LEFT JOIN tbl_middle_left_columnar c ON h1.id = c.id
+LEFT JOIN tbl_middle_left_heap2 h2 ON c.id = h2.id
+ORDER BY 1;
+
 -- End test case
 SET client_min_messages TO warning;
 DROP SCHEMA am_columnar_join CASCADE;