From: "Pengchengliu" <pengchengliu@tju.edu.cn>
To: "'Andres Freund'" <andres@anarazel.de>
Cc: <pgsql-hackers@postgresql.org>
References: 
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In-Reply-To: <000001d7432b$33cc4a00$9b64de00$@tju.edu.cn>
Subject: RE: Parallel scan with SubTransGetTopmostTransaction assert coredump
Date: Tue, 11 May 2021 09:26:23 +0800
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Hi Andres,
   Reproduce steps.

1, Modify and adjust NUM_SUBTRANS_BUFFERS to 128 from 32 in the file  =
"src/include/access/subtrans.h" line number 15.
2, configure with enable assert and build it.
3, init a new database cluster.
4, modify  postgres.conf  and add some parameters as below. As the =
coredump from parallel scan, so we adjust parallel setting, make it easy =
to reproduce.=20

  max_connections =3D 2000

  parallel_setup_cost=3D0
  parallel_tuple_cost=3D0
  min_parallel_table_scan_size=3D0
  max_parallel_workers_per_gather=3D8
  max_parallel_workers =3D 32

5, start the database cluster.
6, use the script init_test.sql  in attachment to create tables.=20
7, use pgbench with script sub_120.sql in attachment to test it. Try it =
sometimes, you should get the coredump file.
    pgbench  -d postgres -p 33550  -n -r -f sub_120.sql   -c 200 -j 200 =
-T 120

Thanks
Pengcheng


-----Original Message-----
From: Andres Freund <andres@anarazel.de>=20
Sent: 2021=E5=B9=B45=E6=9C=887=E6=97=A5 11:55
To: Pengchengliu <pengchengliu@tju.edu.cn>
Cc: pgsql-hackers@postgresql.org
Subject: Re: Parallel scan with SubTransGetTopmostTransaction assert =
coredump

Hi,

On 2021-05-07 11:32:57 +0800, Pengchengliu wrote:
> Hi Hackers=EF=BC=8C
>=20
> Last email, format error, missing some information, so I resend this =
email. =20
>=20
>  With PG 13.2(3fb4c75e857adee3da4386e947ba58a75f3e74b7), I tested =
subtransaction with parallel scan, I got a subtransaction coredump as =
below:


> So the root cause is the Parallel Workers process set the =
TransactionXmin with later transcation snapshot. When parallel scan, =
Parallel Workers process use the older active snapshot.
>=20
> It leads to subtrans assert coredump. I don't know how to fix it. Is =
there any ideas?

Do you have steps to reliably reproduce this?

Greetings,

Andres Freund