Re: Parallel Seq Scan

On Sun, Aug 2, 2015 at 8:06 AM, Kouhei Kaigai <kaigai(at)ak(dot)jp(dot)nec(dot)com> wrote:
>
> Amit,
>
> Let me ask three more detailed questions.
>
> Why Funnel has a valid qual of the subplan?
> The 2nd argument of make_funnel() is qualifier of the subplan
> (PartialSeqScan) then it is initialized at ExecInitFunnel,
> but never executed on the run-time. Why does Funnel node has
> useless qualifier expression here (even though it is harmless)?
>

The idea is that if in some case the qualification can't be
pushed down (consider the case where qualification contains
parallel restricted functions (functions that can only be
executed in master backend)) and needs to be only executed
in master backend, then we need it in Funnel node, so that it
can be executed for tuples passed by worker backends. It is
currently not used, but I think we should retain it as it is
because it can be used in some cases either as part of this
patch itself or in future. As of now, it is used in other
places in patch (like during Explain) as well, although we
might want to optimize the same, but overall I think it is
required.

> Why Funnel delivered from Scan? Even though it constructs
> a compatible target-list with underlying partial-scan node,
> it does not require the node is also delivered from Scan.

It needs it's own target-list due to reason mentioned above
for qual and yet another reason is that the same is required
for FunnelState which inturn is required ScanSlot used to
retrieve tuples from workers. Also it is not excatly same
as partialseqscan, because for the case when the partialseqscan
node is executed by worker, we modify the targetlist as well,
refer create_parallel_worker_plannedstmt().

>
> Does ExecFunnel() need to have a special code path to handle
> EvalPlanQual()? Probably, it just calls underlying node in the
> local context. ExecScan() of PartialSeqScan will check its
> qualifier towards estate->es_epqTuple[].
>

Isn't EvalPlanQual() called for modifytable node and which
won't be allowed in parallel mode, so I think EvalPlanQual()
is not required for ExecFunnel path.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com