Fix thinko in join removal.

In commit 9df8f903e I (tgl) switched join_is_removable() from
using the min relid sets of the join under consideration to
using its full syntactic relid sets.  This was a mistake,
as it allowed join removal in cases where a reference to the
join output would survive in some syntactically-lower join
condition.  Revert to the former coding.

Richard Guo

Discussion: https://postgr.es/m/CAMbWs4-EU9uBGSP7G-iTwLBhRQ=rnZKvFDhD+n+xhajokyPCKg@mail.gmail.com

src/backend/optimizer/plan/analyzejoins.c

@@ -88,11 +88,8 @@ restart:
 		 */
 		innerrelid = bms_singleton_member(sjinfo->min_righthand);
 
-		/*
+		/* Compute the relid set for the join we are considering */
-		 * Compute the relid set for the join we are considering.  We can
+		joinrelids = bms_union(sjinfo->min_lefthand, sjinfo->min_righthand);
-		 * assume things are done in syntactic order.
-		 */
-		joinrelids = bms_union(sjinfo->syn_lefthand, sjinfo->syn_righthand);
 		if (sjinfo->ojrelid != 0)
 			joinrelids = bms_add_member(joinrelids, sjinfo->ojrelid);
 
@@ -204,8 +201,8 @@ join_is_removable(PlannerInfo *root, SpecialJoinInfo *sjinfo)
 	if (!rel_supports_distinctness(root, innerrel))
 		return false;
 
-	/* Compute the syntactic relid set for the join we are considering */
+	/* Compute the relid set for the join we are considering */
-	inputrelids = bms_union(sjinfo->syn_lefthand, sjinfo->syn_righthand);
+	inputrelids = bms_union(sjinfo->min_lefthand, sjinfo->min_righthand);
 	Assert(sjinfo->ojrelid != 0);
 	joinrelids = bms_copy(inputrelids);
 	joinrelids = bms_add_member(joinrelids, sjinfo->ojrelid);

src/test/regress/expected/join.out

@@ -5432,6 +5432,22 @@ select d.* from d left join (select distinct * from b) s
          ->  Seq Scan on d
 (9 rows)
 
+-- join removal is not possible here
+explain (costs off)
+select 1 from a t1
+  left join (a t2 left join a t3 on t2.id = 1) on t2.id = 1;
+                       QUERY PLAN                       
+--------------------------------------------------------
+ Nested Loop Left Join
+   ->  Seq Scan on a t1
+   ->  Materialize
+         ->  Nested Loop Left Join
+               Join Filter: (t2.id = 1)
+               ->  Index Only Scan using a_pkey on a t2
+                     Index Cond: (id = 1)
+               ->  Seq Scan on a t3
+(8 rows)
+
 -- check join removal works when uniqueness of the join condition is enforced
 -- by a UNION
 explain (costs off)

src/test/regress/sql/join.sql

@@ -1958,6 +1958,11 @@ explain (costs off)
 select d.* from d left join (select distinct * from b) s
   on d.a = s.id;
 
+-- join removal is not possible here
+explain (costs off)
+select 1 from a t1
+  left join (a t2 left join a t3 on t2.id = 1) on t2.id = 1;
+
 -- check join removal works when uniqueness of the join condition is enforced
 -- by a UNION
 explain (costs off)