diff --git a/src/backend/optimizer/path/equivclass.c b/src/backend/optimizer/path/equivclass.c
index 9f39f4661a..06f89a6732 100644
--- a/src/backend/optimizer/path/equivclass.c
+++ b/src/backend/optimizer/path/equivclass.c
@@ -1851,6 +1851,21 @@ create_join_clause(PlannerInfo *root,
 												  rightem->em_nullable_relids),
 										ec->ec_min_security);
 
+	/*
+	 * If either EM is a child, force the clause's clause_relids to include
+	 * the relid(s) of the child rel.  In normal cases it would already, but
+	 * not if we are considering appendrel child relations with pseudoconstant
+	 * translated variables (i.e., UNION ALL sub-selects with constant output
+	 * items).  We must do this so that join_clause_is_movable_into() will
+	 * think that the clause should be evaluated at the correct place.
+	 */
+	if (leftem->em_is_child)
+		rinfo->clause_relids = bms_add_members(rinfo->clause_relids,
+											   leftem->em_relids);
+	if (rightem->em_is_child)
+		rinfo->clause_relids = bms_add_members(rinfo->clause_relids,
+											   rightem->em_relids);
+
 	/* Mark the clause as redundant, or not */
 	rinfo->parent_ec = parent_ec;
 
diff --git a/src/backend/optimizer/util/relnode.c b/src/backend/optimizer/util/relnode.c
index 3c75fd56f2..6e1c87a4e2 100644
--- a/src/backend/optimizer/util/relnode.c
+++ b/src/backend/optimizer/util/relnode.c
@@ -1300,6 +1300,7 @@ get_baserel_parampathinfo(PlannerInfo *root, RelOptInfo *baserel,
 	ParamPathInfo *ppi;
 	Relids		joinrelids;
 	List	   *pclauses;
+	List	   *eqclauses;
 	double		rows;
 	ListCell   *lc;
 
@@ -1333,14 +1334,24 @@ get_baserel_parampathinfo(PlannerInfo *root, RelOptInfo *baserel,
 	}
 
 	/*
-	 * Add in joinclauses generated by EquivalenceClasses, too.  (These
-	 * necessarily satisfy join_clause_is_movable_into.)
+	 * Add in joinclauses generated by EquivalenceClasses, too.  In principle
+	 * these should always satisfy join_clause_is_movable_into; but if we are
+	 * below an outer join the clauses might contain Vars that should only be
+	 * evaluated above the join, so we have to check.
 	 */
-	pclauses = list_concat(pclauses,
-						   generate_join_implied_equalities(root,
-															joinrelids,
-															required_outer,
-															baserel));
+	eqclauses = generate_join_implied_equalities(root,
+												 joinrelids,
+												 required_outer,
+												 baserel);
+	foreach(lc, eqclauses)
+	{
+		RestrictInfo *rinfo = (RestrictInfo *) lfirst(lc);
+
+		if (join_clause_is_movable_into(rinfo,
+										baserel->relids,
+										joinrelids))
+			pclauses = lappend(pclauses, rinfo);
+	}
 
 	/* Estimate the number of rows returned by the parameterized scan */
 	rows = get_parameterized_baserel_size(root, baserel, pclauses);
diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
index 867c6d20cc..b356153900 100644
--- a/src/test/regress/expected/join.out
+++ b/src/test/regress/expected/join.out
@@ -5949,6 +5949,37 @@ select * from
  3 | 3
 (6 rows)
 
+-- check for generation of join EC conditions at wrong level (bug #18429)
+explain (costs off)
+select * from (
+  select arrayd.ad, coalesce(c.hundred, 0) as h
+  from unnest(array[1]) as arrayd(ad)
+  left join lateral (
+    select hundred from tenk1 where unique2 = arrayd.ad
+  ) c on true
+) c2
+where c2.h * c2.ad = c2.h * (c2.ad + 1);
+                                              QUERY PLAN                                               
+-------------------------------------------------------------------------------------------------------
+ Nested Loop Left Join
+   Filter: ((COALESCE(tenk1.hundred, 0) * arrayd.ad) = (COALESCE(tenk1.hundred, 0) * (arrayd.ad + 1)))
+   ->  Function Scan on unnest arrayd
+   ->  Index Scan using tenk1_unique2 on tenk1
+         Index Cond: (unique2 = arrayd.ad)
+(5 rows)
+
+select * from (
+  select arrayd.ad, coalesce(c.hundred, 0) as h
+  from unnest(array[1]) as arrayd(ad)
+  left join lateral (
+    select hundred from tenk1 where unique2 = arrayd.ad
+  ) c on true
+) c2
+where c2.h * c2.ad = c2.h * (c2.ad + 1);
+ ad | h 
+----+---
+(0 rows)
+
 -- check the number of columns specified
 SELECT * FROM (int8_tbl i cross join int4_tbl j) ss(a,b,c,d);
 ERROR:  join expression "ss" has 3 columns available but 4 columns specified
diff --git a/src/test/regress/sql/join.sql b/src/test/regress/sql/join.sql
index 1113e98445..11a857041a 100644
--- a/src/test/regress/sql/join.sql
+++ b/src/test/regress/sql/join.sql
@@ -2029,6 +2029,25 @@ select * from
    (select q1.v)
   ) as q2;
 
+-- check for generation of join EC conditions at wrong level (bug #18429)
+explain (costs off)
+select * from (
+  select arrayd.ad, coalesce(c.hundred, 0) as h
+  from unnest(array[1]) as arrayd(ad)
+  left join lateral (
+    select hundred from tenk1 where unique2 = arrayd.ad
+  ) c on true
+) c2
+where c2.h * c2.ad = c2.h * (c2.ad + 1);
+select * from (
+  select arrayd.ad, coalesce(c.hundred, 0) as h
+  from unnest(array[1]) as arrayd(ad)
+  left join lateral (
+    select hundred from tenk1 where unique2 = arrayd.ad
+  ) c on true
+) c2
+where c2.h * c2.ad = c2.h * (c2.ad + 1);
+
 -- check the number of columns specified
 SELECT * FROM (int8_tbl i cross join int4_tbl j) ss(a,b,c,d);