Tom Lane wrote:
In principle it could look something like

    (SubPlan N ($0 := b.oid))

but with a few parameters and a bunch of other stuff on the same line
that would get out of hand. 
The patch I submitted to implement this, hits bogus varno:65001 in get_variable when I explain the following query from the aggregates.sql regression test.

select ten, sum(distinct four) from onek a
group by ten
having exists (select 1 from onek b where sum(distinct a.four) = b.four);

The explain without the subplan argument-patch  shows
regression=# explain verbose select ten, sum(distinct four) from onek a
group by ten
having exists (select 1 from onek b where sum(distinct a.four) = b.four);
                                  QUERY PLAN                                 
------------------------------------------------------------------------------
 GroupAggregate  (cost=197.83..216.52 rows=10 width=8)
   Output: a.ten, sum(DISTINCT a.four)
   Filter: (alternatives: SubPlan 1 or hashed SubPlan 2)
   ->  Sort  (cost=197.83..200.33 rows=1000 width=8)
         Output: a.ten, a.four
         Sort Key: a.ten
         ->  Seq Scan on public.onek a  (cost=0.00..148.00 rows=1000 width=8)
               Output: a.ten, a.four
   SubPlan 1
     ->  Seq Scan on public.onek b  (cost=0.00..150.50 rows=250 width=0)
           Filter: ($0 = b.four)
   SubPlan 2
     ->  Seq Scan on public.onek b  (cost=0.00..148.00 rows=1000 width=4)
           Output: b.four
(14 rows)

The subplan argument list contains a aggref node, with a var argument that has the 65001 varno. Apparently the OUTER varno is set by search_indexed_tlist_for_var and it seems like correct behaviour, see stack trace below.

I'm unsure what is a good approach to solve this problem: let get_variable not give an error in this case?

regards,
Yeb Havinga




#0  search_indexed_tlist_for_var (var=0xe32dc0, itlist=0xe67290, newvarno=65001, rtoffset=0)
    at setrefs.c:1568
#1  0x0000000000697581 in fix_upper_expr_mutator (node=0xe32dc0, context=0x7fffabcde850)
    at setrefs.c:1853
#2  0x0000000000697529 in fix_upper_expr (glob=0xe77178, node=0xe32dc0, subplan_itlist=0xe67290,
    rtoffset=0) at setrefs.c:1839
#3  0x0000000000696b6e in set_upper_references (glob=0xe77178, plan=0xe66f80, rtoffset=0)


postgres=# explain select oid::int + 1,(select oid from pg_class a where a.oid = b.relfilenode and a.relnamespace = b.relnamespace) from pg_class b;
                                        QUERY PLAN                                         ---------------------------------------------------------------------------------------------
Seq Scan on pg_class b  (cost=0.00..2459.64 rows=296 width=12)
  SubPlan 1 ($0 := b.relfilenode, $1 := b.relnamespace)
    ->  Index Scan using pg_class_oid_index on pg_class a  (cost=0.00..8.27 rows=1 width=4)
          Index Cond: (oid = $0)
          Filter: (relnamespace = $1)
(5 rows)

index 56d9c5b..454d59b 100644
--- a/src/backend/commands/explain.c
+++ b/src/backend/commands/explain.c
@@ -1686,20 +1686,47 @@ static void
ExplainSubPlans(List *plans, const char *relationship, ExplainState *es)
{
       ListCell   *lst;

       foreach(lst, plans)
       {
               SubPlanState *sps = (SubPlanState *) lfirst(lst);
-               SubPlan    *sp = (SubPlan *) sps->xprstate.expr;
+               SubPlan      *sp = (SubPlan *) sps->xprstate.expr;
+               StringInfo    signature = makeStringInfo();
+               int           i = 0;
+               List         *context;
+               bool          useprefix;
+               ListCell     *c;
+
+               context = deparse_context_for_plan((Node *)exec_subplan_get_plan(es->pstmt, sp),
+                                                                                  NULL,
+                                                                                  es->rtable,
+                                                                                  es->pstmt->subplans);
+               useprefix = list_length(es->rtable) > 1;
+
+               appendStringInfoString(signature, sp->plan_name);
+
+               foreach(c, sp->args)
+               {
+                       Node *n = lfirst(c);
+                       appendStringInfo(signature, "%s$%d := %s",
+                                                        (i == 0) ? " (" : ", ",
+                                                        i,
+                                                        deparse_expression(n, context, useprefix, true));
+                       i++;
+               }
+
+               if (i > 0)
+                       appendStringInfoString(signature, ")");

               ExplainNode(exec_subplan_get_plan(es->pstmt, sp),
                                       sps->planstate,
                                       NULL,
-                                       relationship, sp->plan_name,
+                                       relationship,
+                                       signature->data,
                                       es);
       }
}

/*
 * Explain a property, such as sort keys or targets, that takes the form of
 * a list of unlabeled items.  "data" is a list of C strings.