From: "Pengchengliu" <pengchengliu@tju.edu.cn>
To: "'Greg Nancarrow'" <gregn4422@gmail.com>
Cc: "'Andres Freund'" <andres@anarazel.de>,
	"'PostgreSQL-development'" <pgsql-hackers@postgresql.org>
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Subject: RE: Re: Parallel scan with SubTransGetTopmostTransaction assert
 coredump
Date: Thu, 20 May 2021 09:18:46 +0800
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Hi Greg,
   Thanks a lot for you explanation and your fix.
  =20
   I think your fix can resolve the core dump issue. As with your fix, =
parallel process reset Transaction Xmin from ActiveSnapshot.=20
   But it will change Transaction snapshot for all parallel  scenarios. =
I don't know whether it bring in other issue.=20
   For test only, I think it is enough.=20

   So is there anybody can explain what's exactly difference between =
ActiveSnapshot and TransactionSnapshot in parallel work process.=20
   Then maybe we can find a better solution and try to fix it really.

Thanks
Pengcheng=20

-----Original Message-----
From: Greg Nancarrow <gregn4422@gmail.com>=20
Sent: 2021=E5=B9=B45=E6=9C=8818=E6=97=A5 17:15
To: Pengchengliu <pengchengliu@tju.edu.cn>
Cc: Andres Freund <andres@anarazel.de>; PostgreSQL-development =
<pgsql-hackers@postgresql.org>
Subject: Re: Re: Parallel scan with SubTransGetTopmostTransaction assert =
coredump

On Tue, May 18, 2021 at 11:27 AM Pengchengliu <pengchengliu@tju.edu.cn> =
wrote:
>
> Hi Greg,
>
>    Actually I am very confused about ActiveSnapshot and =
TransactionSnapshot. I don't know why main process send ActiveSnapshot =
and TransactionSnapshot separately.  And what is exact difference =
between them?
>    If you know that, could you explain that for me? It will be very =
appreciated.

In the context of a parallel-worker, I am a little confused too, so I =
can't explain it either.
It is not really explained in the file
"src\backend\access\transam\README.parallel", it only mentions the =
following as part of the state that needs to be copied to each worker:

 - The transaction snapshot.
 - The active snapshot, which might be different from the transaction =
snapshot.

So they might be different, but exactly when and why?

When I debugged a typical parallel-SELECT case, I found that prior to =
plan execution, GetTransactionSnapshot() was called and its return value =
was stored in both the QueryDesc and the estate (es_snapshot), which was =
then pushed on the ActiveSnapshot stack. So by the time
InitializeParallelDSM() was called, the (top) ActiveSnapshot was the =
last snapshot returned from GetTransactionSnapshot().
So why InitializeParallelDSM() calls GetTransactionSnapshot() again is =
not clear to me (because isn't then the ActiveSnapshot a potentially =
earlier snapshot? - which it shouldn't be, AFAIK. And also, it's then =
different to the non-parallel case).

>    Before we know them exactly, I think we should not modify the =
TransactionSnapshot to ActiveSnapshot in main process. If it is, the =
main process should send ActiveSnapshot only.

I think it would be worth you trying my suggested change (if you have a =
development environment, which I assume you have). Sure, IF it was =
deemed a proper solution, you'd only send the one snapshot, and adjust =
accordingly in ParallelWorkerMain(), but we need not worry about that in =
order to test it.


Regards,
Greg Nancarrow
Fujitsu Australia