Prevent inlining of multiply-referenced CTEs with outer recursive refs.

This has to be prevented because inlining would result in multiple
self-references, which we don't support (and in fact that's disallowed
by the SQL spec, see statements about linearly vs. nonlinearly
recursive queries).  Bug fix for commit 608b167f9.

Per report from Yaroslav Schekin (via Andrew Gierth)

Discussion: https://postgr.es/m/87wolmg60q.fsf@news-spur.riddles.org.uk

src/test/regress/expected/subselect.out

@@ -1299,6 +1299,106 @@ select * from x, x x2 where x.n = x2.n;
                      Output: subselect_tbl_1.f1
 (11 rows)
 
+-- Multiply-referenced CTEs can't be inlined if they contain outer self-refs
+explain (verbose, costs off)
+with recursive x(a) as
+  ((values ('a'), ('b'))
+   union all
+   (with z as not materialized (select * from x)
+    select z.a || z1.a as a from z cross join z as z1
+    where length(z.a || z1.a) < 5))
+select * from x;
+                        QUERY PLAN                        
+----------------------------------------------------------
+ CTE Scan on x
+   Output: x.a
+   CTE x
+     ->  Recursive Union
+           ->  Values Scan on "*VALUES*"
+                 Output: "*VALUES*".column1
+           ->  Nested Loop
+                 Output: (z.a || z1.a)
+                 Join Filter: (length((z.a || z1.a)) < 5)
+                 CTE z
+                   ->  WorkTable Scan on x x_1
+                         Output: x_1.a
+                 ->  CTE Scan on z
+                       Output: z.a
+                 ->  CTE Scan on z z1
+                       Output: z1.a
+(16 rows)
+
+with recursive x(a) as
+  ((values ('a'), ('b'))
+   union all
+   (with z as not materialized (select * from x)
+    select z.a || z1.a as a from z cross join z as z1
+    where length(z.a || z1.a) < 5))
+select * from x;
+  a   
+------
+ a
+ b
+ aa
+ ab
+ ba
+ bb
+ aaaa
+ aaab
+ aaba
+ aabb
+ abaa
+ abab
+ abba
+ abbb
+ baaa
+ baab
+ baba
+ babb
+ bbaa
+ bbab
+ bbba
+ bbbb
+(22 rows)
+
+explain (verbose, costs off)
+with recursive x(a) as
+  ((values ('a'), ('b'))
+   union all
+   (with z as not materialized (select * from x)
+    select z.a || z.a as a from z
+    where length(z.a || z.a) < 5))
+select * from x;
+                       QUERY PLAN                       
+--------------------------------------------------------
+ CTE Scan on x
+   Output: x.a
+   CTE x
+     ->  Recursive Union
+           ->  Values Scan on "*VALUES*"
+                 Output: "*VALUES*".column1
+           ->  WorkTable Scan on x x_1
+                 Output: (x_1.a || x_1.a)
+                 Filter: (length((x_1.a || x_1.a)) < 5)
+(9 rows)
+
+with recursive x(a) as
+  ((values ('a'), ('b'))
+   union all
+   (with z as not materialized (select * from x)
+    select z.a || z.a as a from z
+    where length(z.a || z.a) < 5))
+select * from x;
+  a   
+------
+ a
+ b
+ aa
+ bb
+ aaaa
+ bbbb
+(6 rows)
+
 -- Check handling of outer references
 explain (verbose, costs off)
 with x as (select * from int4_tbl)

src/test/regress/sql/subselect.sql

@@ -690,6 +690,41 @@ explain (verbose, costs off)
 with x as not materialized (select * from (select f1, now() as n from subselect_tbl) ss)
 select * from x, x x2 where x.n = x2.n;
 
+-- Multiply-referenced CTEs can't be inlined if they contain outer self-refs
+explain (verbose, costs off)
+with recursive x(a) as
+  ((values ('a'), ('b'))
+   union all
+   (with z as not materialized (select * from x)
+    select z.a || z1.a as a from z cross join z as z1
+    where length(z.a || z1.a) < 5))
+select * from x;
+
+with recursive x(a) as
+  ((values ('a'), ('b'))
+   union all
+   (with z as not materialized (select * from x)
+    select z.a || z1.a as a from z cross join z as z1
+    where length(z.a || z1.a) < 5))
+select * from x;
+
+explain (verbose, costs off)
+with recursive x(a) as
+  ((values ('a'), ('b'))
+   union all
+   (with z as not materialized (select * from x)
+    select z.a || z.a as a from z
+    where length(z.a || z.a) < 5))
+select * from x;
+
+with recursive x(a) as
+  ((values ('a'), ('b'))
+   union all
+   (with z as not materialized (select * from x)
+    select z.a || z.a as a from z
+    where length(z.a || z.a) < 5))
+select * from x;
+
 -- Check handling of outer references
 explain (verbose, costs off)
 with x as (select * from int4_tbl)