Fix an oversight in checking whether a join with LATERAL refs is legal.

In many cases, we can implement a semijoin as a plain innerjoin by first
passing the righthand-side relation through a unique-ification step.
However, one of the cases where this does NOT work is where the RHS has
a LATERAL reference to the LHS; that makes the RHS dependent on the LHS
so that unique-ification is meaningless.  joinpath.c understood this,
and so would not generate any join paths of this kind ... but join_is_legal
neglected to check for the case, so it would think that we could do it.
The upshot would be a "could not devise a query plan for the given query"
failure once we had failed to generate any join paths at all for the bogus
join pair.

Back-patch to 9.3 where LATERAL was added.

src/backend/optimizer/path/joinrels.c

@@ -536,7 +536,9 @@ join_is_legal(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
 			if (!bms_is_subset(ljinfo->lateral_lhs, rel1->relids))
 				return false;	/* rel1 can't compute the required parameter */
 			if (match_sjinfo &&
-				(reversed || match_sjinfo->jointype == JOIN_FULL))
+				(reversed ||
+				 unique_ified ||
+				 match_sjinfo->jointype == JOIN_FULL))
 				return false;	/* not implementable as nestloop */
 		}
 		if (bms_is_subset(ljinfo->lateral_rhs, rel1->relids) &&
@@ -549,7 +551,9 @@ join_is_legal(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
 			if (!bms_is_subset(ljinfo->lateral_lhs, rel2->relids))
 				return false;	/* rel2 can't compute the required parameter */
 			if (match_sjinfo &&
-				(!reversed || match_sjinfo->jointype == JOIN_FULL))
+				(!reversed ||
+				 unique_ified ||
+				 match_sjinfo->jointype == JOIN_FULL))
 				return false;	/* not implementable as nestloop */
 		}
 	}

src/test/regress/expected/join.out

@@ -4527,6 +4527,41 @@ select * from
          Output: 3
 (11 rows)
 
+-- check we don't try to do a unique-ified semijoin with LATERAL
+explain (verbose, costs off)
+select * from
+  (values (0,9998), (1,1000)) v(id,x),
+  lateral (select f1 from int4_tbl
+           where f1 = any (select unique1 from tenk1
+                           where unique2 = v.x offset 0)) ss;
+                              QUERY PLAN                              
+----------------------------------------------------------------------
+ Nested Loop
+   Output: "*VALUES*".column1, "*VALUES*".column2, int4_tbl.f1
+   ->  Values Scan on "*VALUES*"
+         Output: "*VALUES*".column1, "*VALUES*".column2
+   ->  Nested Loop Semi Join
+         Output: int4_tbl.f1
+         Join Filter: (int4_tbl.f1 = tenk1.unique1)
+         ->  Seq Scan on public.int4_tbl
+               Output: int4_tbl.f1
+         ->  Materialize
+               Output: tenk1.unique1
+               ->  Index Scan using tenk1_unique2 on public.tenk1
+                     Output: tenk1.unique1
+                     Index Cond: (tenk1.unique2 = "*VALUES*".column2)
+(14 rows)
+
+select * from
+  (values (0,9998), (1,1000)) v(id,x),
+  lateral (select f1 from int4_tbl
+           where f1 = any (select unique1 from tenk1
+                           where unique2 = v.x offset 0)) ss;
+ id |  x   | f1 
+----+------+----
+  0 | 9998 |  0
+(1 row)
+
 -- test some error cases where LATERAL should have been used but wasn't
 select f1,g from int4_tbl a, (select f1 as g) ss;
 ERROR:  column "f1" does not exist

src/test/regress/sql/join.sql

@@ -1365,6 +1365,19 @@ select * from
     select * from (select 3 as z offset 0) z where z.z = x.x
   ) zz on zz.z = y.y;
 
+-- check we don't try to do a unique-ified semijoin with LATERAL
+explain (verbose, costs off)
+select * from
+  (values (0,9998), (1,1000)) v(id,x),
+  lateral (select f1 from int4_tbl
+           where f1 = any (select unique1 from tenk1
+                           where unique2 = v.x offset 0)) ss;
+select * from
+  (values (0,9998), (1,1000)) v(id,x),
+  lateral (select f1 from int4_tbl
+           where f1 = any (select unique1 from tenk1
+                           where unique2 = v.x offset 0)) ss;
+
 -- test some error cases where LATERAL should have been used but wasn't
 select f1,g from int4_tbl a, (select f1 as g) ss;
 select f1,g from int4_tbl a, (select a.f1 as g) ss;