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.

1 files changed, 6 insertions, 2 deletions

diff --git a/src/backend/optimizer/path/joinrels.c b/src/backend/optimizer/path/joinrels.c
index 610892890f5..ec0cf1a99a8 100644
--- a/src/backend/optimizer/path/joinrels.c
+++ b/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 */
 		}
 	}