pg.ddx.io pgsql-hackers@postgresql.org mailing list archive help / color / mirror / Atom feed From: Andrey Lepikhov <a.lepikhov@postgrespro.ru>
To: Etsuro Fujita <etsuro.fujita@gmail.com>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Asynchronous Append on postgres_fdw nodes.
Date: Tue, 11 May 2021 07:58:10 +0500
Message-ID: <2080193d-da8d-4132-f48c-8e29be8fcffc@postgrespro.ru> (raw )
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On 10/5/21 08:03, Etsuro Fujita wrote:
> On Fri, May 7, 2021 at 7:32 PM Andrey Lepikhov
> I think a simple fix for this would be just remove the check whether
> the instr->running flag is set or not in InstrUpdateTupleCount().
> Attached is an updated patch, in which I also updated a comment in
> execnodes.h and docs in fdwhandler.sgml to match the code in
> nodeAppend.c, and fixed typos in comments in nodeAppend.c.
Your patch fixes the problem. But I found two more problems:
EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
SELECT * FROM (
(SELECT * FROM f1)
UNION ALL
(SELECT * FROM f2)
UNION ALL
(SELECT *
FROM l3)
) q1
LIMIT 6709;
QUERY PLAN
--------------------------------------------------------------
Limit (actual rows=6709 loops=1)
-> Append (actual rows=6709 loops=1)
-> Async Foreign Scan on f1 (actual rows=1 loops=1)
-> Async Foreign Scan on f2 (actual rows=1 loops=1)
-> Seq Scan on l3 (actual rows=6708 loops=1)
Here we scan 6710 tuples at low level but appended only 6709. Where did
we lose one tuple?
2.
SELECT * FROM (
(SELECT * FROM f1)
UNION ALL
(SELECT * FROM f2)
UNION ALL
(SELECT * FROM f3 WHERE a > 0)
) q1 LIMIT 3000;
QUERY PLAN
--------------------------------------------------------------
Limit (actual rows=3000 loops=1)
-> Append (actual rows=3000 loops=1)
-> Async Foreign Scan on f1 (actual rows=0 loops=1)
-> Async Foreign Scan on f2 (actual rows=0 loops=1)
-> Foreign Scan on f3 (actual rows=3000 loops=1)
Here we give preference to the synchronous scan. Why?
--
regards,
Andrey Lepikhov
Postgres Professional
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To: pgsql-hackers@postgresql.org
Cc: a.lepikhov@postgrespro.ru, etsuro.fujita@gmail.com, horikyota.ntt@gmail.com, pryzby@telsasoft.com, pgsql-hackers@lists.postgresql.org
Subject: Re: Asynchronous Append on postgres_fdw nodes.
In-Reply-To: <2080193d-da8d-4132-f48c-8e29be8fcffc@postgrespro.ru>
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