Re: Parallel Append implementation

Hi Amit,

On 2016/12/23 14:21, Amit Khandekar wrote:
> Currently an Append plan node does not execute its subplans in
> parallel. There is no distribution of workers across its subplans. The
> second subplan starts running only after the first subplan finishes,
> although the individual subplans may be running parallel scans.
> 
> Secondly, we create a partial Append path for an appendrel, but we do
> that only if all of its member subpaths are partial paths. If one or
> more of the subplans is a non-parallel path, there will be only a
> non-parallel Append. So whatever node is sitting on top of Append is
> not going to do a parallel plan; for example, a select count(*) won't
> divide it into partial aggregates if the underlying Append is not
> partial.
> 
> The attached patch removes both of the above restrictions.  There has
> already been a mail thread [1] that discusses an approach suggested by
> Robert Haas for implementing this feature. This patch uses this same
> approach.

I was looking at the executor portion of this patch and I noticed that in
exec_append_initialize_next():

    if (appendstate->as_padesc)
        return parallel_append_next(appendstate);

    /*
     * Not parallel-aware. Fine, just go on to the next subplan in the
     * appropriate direction.
     */
    if (ScanDirectionIsForward(appendstate->ps.state->es_direction))
        appendstate->as_whichplan++;
    else
        appendstate->as_whichplan--;

which seems to mean that executing Append in parallel mode disregards the
scan direction.  I am not immediately sure what implications that has, so
I checked what heap scan does when executing in parallel mode, and found
this in heapgettup():

    else if (backward)
    {
        /* backward parallel scan not supported */
        Assert(scan->rs_parallel == NULL);

Perhaps, AppendState.as_padesc would not have been set if scan direction
is backward, because parallel mode would be disabled for the whole query
in that case (PlannerGlobal.parallelModeOK = false).  Maybe add an
Assert() similar to one in heapgettup().

Thanks,
Amit