Fix old oversight in join removal logic.

Commit 9e7e29c75ad441450f9b8287bd51c13521641e3b introduced an Assert that
join removal didn't reduce the eval_at set of any PlaceHolderVar to empty.
At first glance it looks like join_is_removable ensures that's true --- but
actually, the loop in join_is_removable skips PlaceHolderVars that are not
referenced above the join due to be removed.  So, if we don't want any
empty eval_at sets, the right thing to do is to delete any now-unreferenced
PlaceHolderVars from the data structure entirely.

Per fuzz testing by Andreas Seltenreich.  Back-patch to 9.3 where the
aforesaid Assert was added.

src/backend/optimizer/plan/analyzejoins.c

@@ -464,17 +464,25 @@ remove_rel_from_query(PlannerInfo *root, int relid, Relids joinrelids)
 	}
 
 	/*
-	 * Likewise remove references from PlaceHolderVar data structures.
+	 * Likewise remove references from PlaceHolderVar data structures,
+	 * removing any no-longer-needed placeholders entirely.
 	 */
-	foreach(l, root->placeholder_list)
+	for (l = list_head(root->placeholder_list); l != NULL; l = nextl)
 	{
 		PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(l);
 
+		nextl = lnext(l);
+		if (bms_is_subset(phinfo->ph_needed, joinrelids))
+			root->placeholder_list = list_delete_ptr(root->placeholder_list,
+													 phinfo);
+		else
+		{
 			phinfo->ph_eval_at = bms_del_member(phinfo->ph_eval_at, relid);
 			Assert(!bms_is_empty(phinfo->ph_eval_at));
 			Assert(!bms_is_member(relid, phinfo->ph_lateral));
 			phinfo->ph_needed = bms_del_member(phinfo->ph_needed, relid);
 		}
+	}
 
 	/*
 	 * Remove any joinquals referencing the rel from the joininfo lists.

src/test/regress/expected/join.out

@@ -3862,6 +3862,22 @@ SELECT * FROM
  1 | 4567890123456789 | -4567890123456789 | 4567890123456789
 (5 rows)
 
+rollback;
+-- another join removal bug: we must clean up correctly when removing a PHV
+begin;
+create temp table uniquetbl (f1 text unique);
+explain (costs off)
+select t1.* from
+  uniquetbl as t1
+  left join (select *, '***'::text as d1 from uniquetbl) t2
+  on t1.f1 = t2.f1
+  left join uniquetbl t3
+  on t2.d1 = t3.f1;
+        QUERY PLAN        
+--------------------------
+ Seq Scan on uniquetbl t1
+(1 row)
+
 rollback;
 -- bug #8444: we've historically allowed duplicate aliases within aliased JOINs
 select * from

src/test/regress/sql/join.sql

@@ -1247,6 +1247,21 @@ SELECT * FROM
 
 rollback;
 
+-- another join removal bug: we must clean up correctly when removing a PHV
+begin;
+
+create temp table uniquetbl (f1 text unique);
+
+explain (costs off)
+select t1.* from
+  uniquetbl as t1
+  left join (select *, '***'::text as d1 from uniquetbl) t2
+  on t1.f1 = t2.f1
+  left join uniquetbl t3
+  on t2.d1 = t3.f1;
+
+rollback;
+
 -- bug #8444: we've historically allowed duplicate aliases within aliased JOINs
 
 select * from