diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index 08a049232e..530328af43 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -2352,6 +2352,7 @@ _outForeignKeyOptInfo(StringInfo str, const ForeignKeyOptInfo *node)
 	WRITE_ATTRNUMBER_ARRAY(confkey, node->nkeys);
 	WRITE_OID_ARRAY(conpfeqop, node->nkeys);
 	WRITE_INT_FIELD(nmatched_ec);
+	WRITE_INT_FIELD(nconst_ec);
 	WRITE_INT_FIELD(nmatched_rcols);
 	WRITE_INT_FIELD(nmatched_ri);
 	/* for compactness, just print the number of matches per column: */
diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index 733f7ea543..9f6507eacb 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -5066,9 +5066,16 @@ get_foreign_key_join_selectivity(PlannerInfo *root,
 		 * remove back into the worklist.
 		 *
 		 * Since the matching clauses are known not outerjoin-delayed, they
-		 * should certainly have appeared in the initial joinclause list.  If
-		 * we didn't find them, they must have been matched to, and removed
-		 * by, some other FK in a previous iteration of this loop.  (A likely
+		 * would normally have appeared in the initial joinclause list.  If we
+		 * didn't find them, there are two possibilities:
+		 *
+		 * 1. If the FK match is based on an EC that is ec_has_const, it won't
+		 * have generated any join clauses at all.  We discount such ECs while
+		 * checking to see if we have "all" the clauses.  (Below, we'll adjust
+		 * the selectivity estimate for this case.)
+		 *
+		 * 2. The clauses were matched to some other FK in a previous
+		 * iteration of this loop, and thus removed from worklist.  (A likely
 		 * case is that two FKs are matched to the same EC; there will be only
 		 * one EC-derived clause in the initial list, so the first FK will
 		 * consume it.)  Applying both FKs' selectivity independently risks
@@ -5079,7 +5086,7 @@ get_foreign_key_join_selectivity(PlannerInfo *root,
 		 * but for now, just punt, since this is a fairly uncommon situation.
 		 */
 		if (list_length(removedlist) !=
-			(fkinfo->nmatched_ec + fkinfo->nmatched_ri))
+			(fkinfo->nmatched_ec - fkinfo->nconst_ec + fkinfo->nmatched_ri))
 		{
 			worklist = list_concat(worklist, removedlist);
 			continue;
@@ -5138,9 +5145,48 @@ get_foreign_key_join_selectivity(PlannerInfo *root,
 
 			fkselec *= 1.0 / ref_tuples;
 		}
+
+		/*
+		 * If any of the FK columns participated in ec_has_const ECs, then
+		 * equivclass.c will have generated "var = const" restrictions for
+		 * each side of the join, thus reducing the sizes of both input
+		 * relations.  Taking the fkselec at face value would amount to
+		 * double-counting the selectivity of the constant restriction for the
+		 * referencing Var.  Hence, look for the restriction clause(s) that
+		 * were applied to the referencing Var(s), and divide out their
+		 * selectivity to correct for this.
+		 */
+		if (fkinfo->nconst_ec > 0)
+		{
+			for (int i = 0; i < fkinfo->nkeys; i++)
+			{
+				EquivalenceClass *ec = fkinfo->eclass[i];
+
+				if (ec && ec->ec_has_const)
+				{
+					EquivalenceMember *em = fkinfo->fk_eclass_member[i];
+					RestrictInfo *rinfo = find_derived_clause_for_ec_member(ec,
+																			em);
+
+					if (rinfo)
+					{
+						Selectivity s0;
+
+						s0 = clause_selectivity(root,
+												(Node *) rinfo,
+												0,
+												jointype,
+												sjinfo);
+						if (s0 > 0)
+							fkselec /= s0;
+					}
+				}
+			}
+		}
 	}
 
 	*restrictlist = worklist;
+	CLAMP_PROBABILITY(fkselec);
 	return fkselec;
 }
 
diff --git a/src/backend/optimizer/path/equivclass.c b/src/backend/optimizer/path/equivclass.c
index 26c98198b5..a21b3b4756 100644
--- a/src/backend/optimizer/path/equivclass.c
+++ b/src/backend/optimizer/path/equivclass.c
@@ -2183,6 +2183,10 @@ exprs_known_equal(PlannerInfo *root, Node *item1, Node *item2)
  * we ignore that fine point here.)  This is much like exprs_known_equal,
  * except that we insist on the comparison operator matching the eclass, so
  * that the result is definite not approximate.
+ *
+ * On success, we also set fkinfo->eclass[colno] to the matching eclass,
+ * and set fkinfo->fk_eclass_member[colno] to the eclass member for the
+ * referencing Var.
  */
 EquivalenceClass *
 match_eclasses_to_foreign_key_col(PlannerInfo *root,
@@ -2212,8 +2216,8 @@ match_eclasses_to_foreign_key_col(PlannerInfo *root,
 	{
 		EquivalenceClass *ec = (EquivalenceClass *) list_nth(root->eq_classes,
 															 i);
-		bool		item1member = false;
-		bool		item2member = false;
+		EquivalenceMember *item1_em = NULL;
+		EquivalenceMember *item2_em = NULL;
 		ListCell   *lc2;
 
 		/* Never match to a volatile EC */
@@ -2238,12 +2242,12 @@ match_eclasses_to_foreign_key_col(PlannerInfo *root,
 
 			/* Match? */
 			if (var->varno == var1varno && var->varattno == var1attno)
-				item1member = true;
+				item1_em = em;
 			else if (var->varno == var2varno && var->varattno == var2attno)
-				item2member = true;
+				item2_em = em;
 
 			/* Have we found both PK and FK column in this EC? */
-			if (item1member && item2member)
+			if (item1_em && item2_em)
 			{
 				/*
 				 * Succeed if eqop matches EC's opfamilies.  We could test
@@ -2253,7 +2257,11 @@ match_eclasses_to_foreign_key_col(PlannerInfo *root,
 				if (opfamilies == NIL)	/* compute if we didn't already */
 					opfamilies = get_mergejoin_opfamilies(eqop);
 				if (equal(opfamilies, ec->ec_opfamilies))
+				{
+					fkinfo->eclass[colno] = ec;
+					fkinfo->fk_eclass_member[colno] = item2_em;
 					return ec;
+				}
 				/* Otherwise, done with this EC, move on to the next */
 				break;
 			}
@@ -2262,6 +2270,37 @@ match_eclasses_to_foreign_key_col(PlannerInfo *root,
 	return NULL;
 }
 
+/*
+ * find_derived_clause_for_ec_member
+ *	  Search for a previously-derived clause mentioning the given EM.
+ *
+ * The eclass should be an ec_has_const EC, of which the EM is a non-const
+ * member.  This should ensure there is just one derived clause mentioning
+ * the EM (and equating it to a constant).
+ * Returns NULL if no such clause can be found.
+ */
+RestrictInfo *
+find_derived_clause_for_ec_member(EquivalenceClass *ec,
+								  EquivalenceMember *em)
+{
+	ListCell   *lc;
+
+	Assert(ec->ec_has_const);
+	Assert(!em->em_is_const);
+	foreach(lc, ec->ec_derives)
+	{
+		RestrictInfo *rinfo = (RestrictInfo *) lfirst(lc);
+
+		/*
+		 * generate_base_implied_equalities_const will have put non-const
+		 * members on the left side of derived clauses.
+		 */
+		if (rinfo->left_em == em)
+			return rinfo;
+	}
+	return NULL;
+}
+
 
 /*
  * add_child_rel_equivalences
diff --git a/src/backend/optimizer/plan/initsplan.c b/src/backend/optimizer/plan/initsplan.c
index d64b32d4ba..aae5df09f9 100644
--- a/src/backend/optimizer/plan/initsplan.c
+++ b/src/backend/optimizer/plan/initsplan.c
@@ -2512,18 +2512,19 @@ match_foreign_keys_to_quals(PlannerInfo *root)
 		 */
 		for (colno = 0; colno < fkinfo->nkeys; colno++)
 		{
+			EquivalenceClass *ec;
 			AttrNumber	con_attno,
 						ref_attno;
 			Oid			fpeqop;
 			ListCell   *lc2;
 
-			fkinfo->eclass[colno] = match_eclasses_to_foreign_key_col(root,
-																	  fkinfo,
-																	  colno);
+			ec = match_eclasses_to_foreign_key_col(root, fkinfo, colno);
 			/* Don't bother looking for loose quals if we got an EC match */
-			if (fkinfo->eclass[colno] != NULL)
+			if (ec != NULL)
 			{
 				fkinfo->nmatched_ec++;
+				if (ec->ec_has_const)
+					fkinfo->nconst_ec++;
 				continue;
 			}
 
diff --git a/src/backend/optimizer/util/plancat.c b/src/backend/optimizer/util/plancat.c
index f9d0d67aa7..4f0da51c26 100644
--- a/src/backend/optimizer/util/plancat.c
+++ b/src/backend/optimizer/util/plancat.c
@@ -567,9 +567,11 @@ get_relation_foreign_keys(PlannerInfo *root, RelOptInfo *rel,
 			memcpy(info->conpfeqop, cachedfk->conpfeqop, sizeof(info->conpfeqop));
 			/* zero out fields to be filled by match_foreign_keys_to_quals */
 			info->nmatched_ec = 0;
+			info->nconst_ec = 0;
 			info->nmatched_rcols = 0;
 			info->nmatched_ri = 0;
 			memset(info->eclass, 0, sizeof(info->eclass));
+			memset(info->fk_eclass_member, 0, sizeof(info->fk_eclass_member));
 			memset(info->rinfos, 0, sizeof(info->rinfos));
 
 			root->fkey_list = lappend(root->fkey_list, info);
diff --git a/src/include/nodes/pathnodes.h b/src/include/nodes/pathnodes.h
index 3dd16b9ad5..45cbf6045a 100644
--- a/src/include/nodes/pathnodes.h
+++ b/src/include/nodes/pathnodes.h
@@ -889,10 +889,13 @@ typedef struct ForeignKeyOptInfo
 
 	/* Derived info about whether FK's equality conditions match the query: */
 	int			nmatched_ec;	/* # of FK cols matched by ECs */
+	int			nconst_ec;		/* # of these ECs that are ec_has_const */
 	int			nmatched_rcols; /* # of FK cols matched by non-EC rinfos */
 	int			nmatched_ri;	/* total # of non-EC rinfos matched to FK */
 	/* Pointer to eclass matching each column's condition, if there is one */
 	struct EquivalenceClass *eclass[INDEX_MAX_KEYS];
+	/* Pointer to eclass member for the referencing Var, if there is one */
+	struct EquivalenceMember *fk_eclass_member[INDEX_MAX_KEYS];
 	/* List of non-EC RestrictInfos matching each column's condition */
 	List	   *rinfos[INDEX_MAX_KEYS];
 } ForeignKeyOptInfo;
diff --git a/src/include/optimizer/paths.h b/src/include/optimizer/paths.h
index 10b6e81079..2134227ebc 100644
--- a/src/include/optimizer/paths.h
+++ b/src/include/optimizer/paths.h
@@ -149,6 +149,8 @@ extern bool exprs_known_equal(PlannerInfo *root, Node *item1, Node *item2);
 extern EquivalenceClass *match_eclasses_to_foreign_key_col(PlannerInfo *root,
 														   ForeignKeyOptInfo *fkinfo,
 														   int colno);
+extern RestrictInfo *find_derived_clause_for_ec_member(EquivalenceClass *ec,
+													   EquivalenceMember *em);
 extern void add_child_rel_equivalences(PlannerInfo *root,
 									   AppendRelInfo *appinfo,
 									   RelOptInfo *parent_rel,