From: Andrey Lepikhov <a.lepikhov@postgrespro.ru>
Subject: Re: Asynchronous Append on postgres_fdw nodes.
To: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: etsuro.fujita@gmail.com, movead.li@highgo.ca,
 pgsql-hackers@lists.postgresql.org
References: 
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 <CAPmGK161L4tgZ5vRrLtA+rmugKbtCq-9o6YhYA+s7oWG_FQ3dg@mail.gmail.com>
 <c5b4ca1e-5550-a405-4199-191143f4e8c2@postgrespro.ru>
 <20200604.150015.2305436578805492574.horikyota.ntt@gmail.com>
Message-ID: <90cced83-9061-0cac-0012-84e5faac8c78@postgrespro.ru>
Date: Tue, 9 Jun 2020 14:20:42 +0500
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On 6/4/20 11:00 AM, Kyotaro Horiguchi wrote:
> Removed a useless variable PgFdwScanState.result_ready.
> Removed duplicate code from remove_async_node() by using move_to_next_waiter().
> Done some minor cleanups.
> 
I am reviewing your code.
A couple of variables are no longer needed (see changes.patch in attachment.

Something about the cost of an asynchronous plan:

At the simple query plan (see below) I see:
1. Startup cost of local SeqScan is equal 0, ForeignScan - 100. But 
startup cost of Append is 0.
2. Total cost of an Append node is a sum of the subplans. Maybe in the 
case of asynchronous append we need to use some reduce factor?

explain select * from parts;

With Async Append:
=====================

  Append  (cost=0.00..2510.30 rows=106780 width=8)
    Async subplans: 3
    ->  Async Foreign Scan on part_1 parts_2  (cost=100.00..177.80 
rows=2260 width=8)
    ->  Async Foreign Scan on part_2 parts_3  (cost=100.00..177.80 
rows=2260 width=8)
    ->  Async Foreign Scan on part_3 parts_4  (cost=100.00..177.80 
rows=2260 width=8)
    ->  Seq Scan on part_0 parts_1  (cost=0.00..1443.00 rows=100000 width=8)

Without Async Append:
=====================

  Append  (cost=0.00..2510.30 rows=106780 width=8)
    ->  Seq Scan on part_0 parts_1  (cost=0.00..1443.00 rows=100000 width=8)
    ->  Foreign Scan on part_1 parts_2  (cost=100.00..177.80 rows=2260 
width=8)
    ->  Foreign Scan on part_2 parts_3  (cost=100.00..177.80 rows=2260 
width=8)
    ->  Foreign Scan on part_3 parts_4  (cost=100.00..177.80 rows=2260 
width=8)

-- 
Andrey Lepikhov
Postgres Professional
https://postgrespro.com
The Russian Postgres Company

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diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c
index b04b6a0e54..4406a9c3b3 100644
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -1632,9 +1632,6 @@ remove_async_node(ForeignScanState *node)
 
 	if (leader == node)
 	{
-		/* It's the leader */
-		ForeignScanState	*next_leader;
-
 		if (leader_state->s.commonstate->busy)
 		{
 			/*
@@ -1769,7 +1766,7 @@ postgresIterateForeignScan(ForeignScanState *node)
 			node->ss.ps.asyncstate = AS_AVAILABLE;
 		else
 			node->ss.ps.asyncstate = AS_WAITING;
-
+elog(WARNING, "No tuple result %d", fsstate->cursor_number);
 		return ExecClearTuple(slot);
 	}
 
@@ -3703,7 +3700,7 @@ request_more_data(ForeignScanState *node)
 
 	snprintf(sql, sizeof(sql), "FETCH %d FROM c%u",
 			 fsstate->fetch_size, fsstate->cursor_number);
-
+elog(WARNING, "FETCH: %s", sql);
 	if (!PQsendQuery(conn, sql))
 		pgfdw_report_error(ERROR, NULL, conn, false, sql);
 
@@ -3769,7 +3766,6 @@ fetch_received_data(ForeignScanState *node)
 	PG_TRY();
 	{
 		PGconn	   *conn = fsstate->s.conn;
-		char		sql[64];
 		int			addrows;
 		size_t		newsize;
 		int			i;
@@ -3798,7 +3794,7 @@ fetch_received_data(ForeignScanState *node)
 										   node,
 										   fsstate->temp_cxt);
 		}
-
+elog(WARNING, "fetch cursor: %d (%d %d)", fsstate->cursor_number, fsstate->num_tuples, addrows);
 		/* Update fetch_ct_2 */
 		if (fsstate->fetch_ct_2 < 2 && fsstate->next_tuple == 0)
 			fsstate->fetch_ct_2++;

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