2 files changed, 69 insertions, 0 deletions

diff --git a/src/backend/optimizer/path/joinrels.c b/src/backend/optimizer/path/joinrels.c
index b2cc9f07f56..9f0212fad23 100644
--- a/src/backend/optimizer/path/joinrels.c
+++ b/src/backend/optimizer/path/joinrels.c
@@ -334,6 +334,7 @@ join_is_legal(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
 	bool		must_be_leftjoin;
 	bool		lateral_fwd;
 	bool		lateral_rev;
+	Relids		join_lateral_rels;
 	ListCell   *l;
 
 	/*
@@ -569,6 +570,35 @@ join_is_legal(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
 		}
 	}
 
+	/*
+	 * LATERAL references could also cause problems later on if we accept this
+	 * join: if the join's minimum parameterization includes any rels that
+	 * would have to be on the inside of an outer join with this join rel,
+	 * then it's never going to be possible to build the complete query using
+	 * this join.  We should reject this join not only because it'll save
+	 * work, but because if we don't, the clauseless-join heuristics might
+	 * think that legality of this join means that some other join rel need
+	 * not be formed, and that could lead to failure to find any plan at all.
+	 * It seems best not to merge this check into the main loop above, because
+	 * it is concerned with SJs that are not otherwise relevant to this join.
+	 */
+	join_lateral_rels = min_join_parameterization(root, joinrelids);
+	if (join_lateral_rels)
+	{
+		foreach(l, root->join_info_list)
+		{
+			SpecialJoinInfo *sjinfo = (SpecialJoinInfo *) lfirst(l);
+
+			if (bms_overlap(sjinfo->min_righthand, join_lateral_rels) &&
+				bms_overlap(sjinfo->min_lefthand, joinrelids))
+				return false;	/* will not be able to join to min_righthand */
+			if (sjinfo->jointype == JOIN_FULL &&
+				bms_overlap(sjinfo->min_lefthand, join_lateral_rels) &&
+				bms_overlap(sjinfo->min_righthand, joinrelids))
+				return false;	/* will not be able to join to min_lefthand */
+		}
+	}
+
 	/* Otherwise, it's a valid join */
 	*sjinfo_p = match_sjinfo;
 	*reversed_p = reversed;
diff --git a/src/backend/optimizer/util/relnode.c b/src/backend/optimizer/util/relnode.c
index 68a93a1a5bd..58b995bd13c 100644
--- a/src/backend/optimizer/util/relnode.c
+++ b/src/backend/optimizer/util/relnode.c
@@ -479,6 +479,45 @@ build_join_rel(PlannerInfo *root,
 }
 
 /*
+ * min_join_parameterization
+ *
+ * Determine the minimum possible parameterization of a joinrel, that is, the
+ * set of other rels it contains LATERAL references to.
+ */
+Relids
+min_join_parameterization(PlannerInfo *root, Relids joinrelids)
+{
+	Relids		result;
+	ListCell   *lc;
+
+	/* Easy if there are no lateral references */
+	if (root->lateral_info_list == NIL)
+		return NULL;
+
+	/*
+	 * Scan lateral_info_list to find all the lateral references occurring in
+	 * or below this join.
+	 */
+	result = NULL;
+	foreach(lc, root->lateral_info_list)
+	{
+		LateralJoinInfo *ljinfo = (LateralJoinInfo *) lfirst(lc);
+
+		if (bms_is_subset(ljinfo->lateral_rhs, joinrelids))
+			result = bms_add_members(result, ljinfo->lateral_lhs);
+	}
+
+	/* Remove any rels that are already included in the join */
+	result = bms_del_members(result, joinrelids);
+
+	/* Maintain invariant that result is exactly NULL if empty */
+	if (bms_is_empty(result))
+		result = NULL;
+
+	return result;
+}
+
+/*
  * build_joinrel_tlist
  *	  Builds a join relation's target list from an input relation.
  *	  (This is invoked twice to handle the two input relations.)