Re: Parallel INSERT (INTO ... SELECT ...)

On Fri, Oct 9, 2020 at 8:09 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> + /*
> + * We need to avoid an attempt on INSERT to assign a
> + * FullTransactionId whilst in parallel mode (which is in
> + * effect due to the underlying parallel query) - so the
> + * FullTransactionId is assigned here. Parallel mode must
> + * be temporarily escaped in order for this to be possible.
> + * The FullTransactionId will be included in the transaction
> + * state that is serialized in the parallel DSM.
> + */
> + if (estate->es_plannedstmt->commandType == CMD_INSERT)
> + {
> + Assert(IsInParallelMode());
> + ExitParallelMode();
> + GetCurrentFullTransactionId();
> + EnterParallelMode();
> + }
> +
>
> This looks like a hack to me. I think you are doing this to avoid the
> parallel mode checks in GetNewTransactionId(), right? If so, I have
> already mentioned above [1] that we can change it so that we disallow
> assigning xids for parallel workers only. The same is true for the
> check in ExecGatherMerge. Do you see any problem with that suggestion?
>

Actually, there is a problem.
If I remove that "hack", and change the code in GetNewTransactionId()
to disallow xid assignment for parallel workers only, then there is
also similar code in AssignTransactionId() which gets called. If I
change that code too, in the same way, then on a parallel INSERT, that
code gets called by a parallel worker (from GetCurrentTransactionId())
and the ERROR "cannot assign XIDs in a parallel worker" results.
GetCurrentFullTransactionId() must be called in the leader, somewhere
(and will be included in the transaction state that is serialized in
the parallel DSM).
If not done here, then where?

Regards,
Greg Nancarrow
Fujitsu Australia