Re: Parallel Inserts in CREATE TABLE AS

On Thu, Dec 10, 2020 at 4:49 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> > +               allow = ps && IsA(ps, GatherState) && !ps->ps_ProjInfo &&
> > +                               plannedstmt->parallelModeNeeded &&
> > +                               plannedstmt->planTree &&
> > +                               IsA(plannedstmt->planTree, Gather) &&
> > +                               plannedstmt->planTree->lefttree &&
> > +                               plannedstmt->planTree->lefttree->parallel_aware &&
> > +                               plannedstmt->planTree->lefttree->parallel_safe;
> >
> > I noticed it check both IsA(ps, GatherState) and IsA(plannedstmt->planTree, Gather).
> > Does it mean it is possible that IsA(ps, GatherState) is true but IsA(plannedstmt->planTree, Gather) is false ?
> >
> > I did some test but did not find a case like that.
> >
>
> This seems like an extra check.  Apart from that if we combine 0001
> and 0002 there should be an additional protection so that it should
> not happen that in cost_gather we have ignored the parallel tuple cost
> and now we are rejecting the parallel insert. Probably we should add
> an assert.

Yeah it's an extra check. I don't think we need that extra check IsA(plannedstmt->planTree, Gather). GatherState check is enough. I verified it as follows: the gatherstate will be allocated and initialized with the plan tree in ExecInitGather which are the ones we are checking here. So, there is no chance that the plan state is GatherState and the plan tree will not be Gather.  I will remove IsA(plannedstmt->planTree, Gather) check in the next version of the patch set.

(gdb) p gatherstate
$10 = (GatherState *) 0x5647fac83850
(gdb) p gatherstate->ps.plan
$11 = (Plan *) 0x5647fac918a0

Breakpoint 1, IsParallelInsertInCTASAllowed (into=0x5647fac97580, queryDesc=0x5647fac835e0) at createas.c:663
663     {
(gdb) p ps
$13 = (PlanState *) 0x5647fac83850
(gdb) p ps->plan
$14 = (Plan *) 0x5647fac918a0

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com