Subject: Re: Asynchronous Append on postgres_fdw nodes.
To: Etsuro Fujita <etsuro.fujita@gmail.com>,
 Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>,
 PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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 <CAPmGK178d9-+aFgYorOY4n0pZmZ55WpQ7i1JcCagovYzAh+ueg@mail.gmail.com>
 <20210331.101137.148208859959175708.horikyota.ntt@gmail.com>
 <CAPmGK17T9Swy2FHZk4AVVKNzRxVnd96PX-18DSC2jeDXWNiBoQ@mail.gmail.com>
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From: "Andrey V. Lepikhov" <a.lepikhov@postgrespro.ru>
Organization: Postgres Professional
Message-ID: <2eb662bb-105d-fc20-7412-2f027cc3ca72@postgrespro.ru>
Date: Mon, 26 Apr 2021 15:35:53 +0500
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On 4/23/21 8:12 AM, Etsuro Fujita wrote:
> I have committed the patch.
Small mistake i found. If no tuple was received from a foreign 
partition, explain shows that we never executed node. For example,
if we have 0 tuples in f1 and 100 tuples in f2:

Query:
EXPLAIN (ANALYZE, VERBOSE, TIMING OFF, COSTS OFF)
SELECT * FROM (SELECT * FROM f1 UNION ALL SELECT * FROM f2) AS q1
LIMIT 101;

Explain:
  Limit (actual rows=100 loops=1)
    Output: f1.a
    ->  Append (actual rows=100 loops=1)
          ->  Async Foreign Scan on public.f1 (never executed)
                Output: f1.a
                Remote SQL: SELECT a FROM public.l1
          ->  Async Foreign Scan on public.f2 (actual rows=100 loops=1)
                Output: f2.a
                Remote SQL: SELECT a FROM public.l2

The patch in the attachment fixes this.

-- 
regards,
Andrey Lepikhov
Postgres Professional

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diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c
index e201b5404e..a960ada441 100644
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -6857,8 +6857,13 @@ produce_tuple_asynchronously(AsyncRequest *areq, bool fetch)
 		}
 		else
 		{
-			/* There's nothing more to do; just return a NULL pointer */
-			result = NULL;
+			/*
+			 * There's nothing more to do; just check it and get an empty slot
+			 * from the child node.
+			 */
+			result = ExecProcNode((PlanState *) node);
+			Assert(TupIsNull(result));
+
 			/* Mark the request as complete */
 			ExecAsyncRequestDone(areq, result);
 		}

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