Re: Parallel Seq Scan

On Thu, Sep 24, 2015 at 12:00 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Thu, Sep 3, 2015 at 6:21 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
wrote:
> > [ new patches ]
>
>
> It looks to me like there would be trouble if an initPlan or subPlan
> were kept below a Funnel, or as I guess we're going to call it, a
> Gather node. That's because a SubPlan doesn't actually have a pointer
> to the node tree for the sub-plan in it. It just has an index into
> PlannedStmt.subplans. But create_parallel_worker_plannedstmt sets the
> subplans list to NIL. So that's not gonna work. Now maybe there's no
> way for an initPlan or a subPlan to creep down under the funnel, but I
> don't immediately see what would prevent it.
>

I think initPlan will work with the existing patches as we are always
executing it in master and then sending the result to workers. Refer
below code in funnel patch:

ExecFunnel()
{
..

+ /*

+ * Evaluate the InitPlan and pass the PARAM_EXEC params, so that

+ * values can be shared with worker backend. This is different from

+ * the way InitPlans are evaluated (lazy evaluation) at other places

+ * as instead of sharing the InitPlan to all the workers and let them

+ * execute, we pass the values which can be directly used by worker

+ * backends.

+ */

+ serialized_param_exec = ExecAndFormSerializeParamExec(econtext,

+ node->ss.ps.plan->lefttree->allParam);
}

For Subplan, as mentioned in yesterday's mail it is still to be dealt
by patch, but I think if we assign the subplans to the planned statement
we are passing to worker, it should work. Here we need to avoid un-safe
subplans to be pushed down, some more thoughts are required to see
what exactly needs to be done.

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