Fix LATERAL references in GRAPH_TABLE with multi-label pattern

When rewriting a GRAPH_TABLE into a subquery,
replace_property_refs_mutator() bumps levelsup of lateral references
by one so that they reference outside the subquery.  This works for
path patterns that result in only one path query.  Patterns that
produce multiple path queries are rewritten as a UNION of path
queries.  Since every path query becomes a subquery of the UNION
statement, the levelsup of lateral references in those path queries
need an additional bump.  This adjustment was missing resulting in the
lateral references being interpreted as references in the UNION query
itself.  This caused different symptoms like a crash in
create_lateral_join_info() or error "plan should not reference
subplan's variable".  The symptoms varied depending on the number of
RangeTblEntries in the UNION statement.

This commit adds the missing adjustment.

Author: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Author: Satya Narlapuram <satyanarlapuram@gmail.com>
Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAHg%2BQDfnLzsgjaQ_CiKSpP4JH3MKOiwoawEcCzXa9uYr45yiWw%40mail.gmail.com

src/backend/rewrite/rewriteGraphTable.c

@@ -714,6 +714,13 @@ generate_setop_from_pathqueries(List *pathqueries, List **rtable, List **targetl
 
 	lquery = linitial_node(Query, pathqueries);
 
+	/*
+	 * Each path query will become a subquery of the UNION statement. So any
+	 * Vars that already refer outside the path query must be adjusted for
+	 * additional query level.
+	 */
+	IncrementVarSublevelsUp((Node *) lquery, 1, 1);
+
 	pni = addRangeTableEntryForSubquery(make_parsestate(NULL), lquery, NULL,
 										false, false);
 	*rtable = lappend(*rtable, pni->p_rte);

src/test/regress/expected/graph_table.out

@@ -254,6 +254,20 @@ SELECT x1.a, g.* FROM x1, GRAPH_TABLE (myshop MATCH (x1 IS customers WHERE x1.ad
  2 | customer1     |   1 |        1
 (2 rows)
 
+-- lateral reference with multi-label pattern, which is rewritten as UNION of
+-- path queries
+SELECT x1.a, g.* FROM x1,
+    GRAPH_TABLE (myshop MATCH (c IS customers WHERE c.customer_id = x1.a)-[IS customer_orders | customer_wishlists]->(l IS lists)-[IS list_items]->(p IS products)
+        COLUMNS (x1.a AS outer_id, c.name AS customer_name, p.name AS product_name, l.list_type)) g
+    ORDER BY 1, 3, 4, 5;
+ a | outer_id | customer_name | product_name | list_type 
+---+----------+---------------+--------------+-----------
+ 1 |        1 | customer1     | product1     | order
+ 1 |        1 | customer1     | product2     | order
+ 2 |        2 | customer2     | product1     | order
+ 2 |        2 | customer2     | product1     | wishlist
+(4 rows)
+
 -- non-local property references are not allowed, even if a lateral column
 -- reference is available
 SELECT x1.a, g.* FROM x1, GRAPH_TABLE (myshop MATCH (x1 IS customers)-[IS customer_orders]->(o IS orders WHERE o.order_id = x1.a) COLUMNS (x1.name AS customer_name, x1.customer_id AS cid, o.order_id)) g; -- error

src/test/regress/sql/graph_table.sql

@@ -158,6 +158,12 @@ CREATE TABLE x1 (a int, address text);
 INSERT INTO x1 VALUES (1, 'one'), (2, 'two');
 SELECT * FROM x1, GRAPH_TABLE (myshop MATCH (c IS customers WHERE c.address = 'US' AND c.customer_id = x1.a)-[IS customer_orders]->(o IS orders) COLUMNS (c.name AS customer_name, c.customer_id AS cid));
 SELECT x1.a, g.* FROM x1, GRAPH_TABLE (myshop MATCH (x1 IS customers WHERE x1.address = 'US')-[IS customer_orders]->(o IS orders) COLUMNS (x1.name AS customer_name, x1.customer_id AS cid, o.order_id)) g;
+-- lateral reference with multi-label pattern, which is rewritten as UNION of
+-- path queries
+SELECT x1.a, g.* FROM x1,
+    GRAPH_TABLE (myshop MATCH (c IS customers WHERE c.customer_id = x1.a)-[IS customer_orders | customer_wishlists]->(l IS lists)-[IS list_items]->(p IS products)
+        COLUMNS (x1.a AS outer_id, c.name AS customer_name, p.name AS product_name, l.list_type)) g
+    ORDER BY 1, 3, 4, 5;
 -- non-local property references are not allowed, even if a lateral column
 -- reference is available
 SELECT x1.a, g.* FROM x1, GRAPH_TABLE (myshop MATCH (x1 IS customers)-[IS customer_orders]->(o IS orders WHERE o.order_id = x1.a) COLUMNS (x1.name AS customer_name, x1.customer_id AS cid, o.order_id)) g; -- error