Re: Parallel Inserts in CREATE TABLE AS - Mailing list pgsql-hackers

On Thu, Dec 10, 2020 at 5:19 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
checkis enough. I verified it as follows: the gatherstate will be allocated and initialized with the plan tree in
ExecInitGatherwhich are the ones we are checking here. So, there is no chance that the plan state is GatherState and
theplan tree will not be Gather.  I will remove IsA(plannedstmt->planTree, Gather) check in the next version of the
patchset. 
> >
> > Breakpoint 4, ExecInitGather (node=0x5647f98ae994 <ExecCheckRTEPerms+131>, estate=0x1ca8, eflags=730035099) at
nodeGather.c:61
> > (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
> >
> Hope you did not miss the second part of my comment
> "
> > Apart from that if we combine 0001
> > and 0002 there should be 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.
> "

IIUC, we need to set a flag in cost_gather(in 0002 patch) whenever we
ignore the parallel tuple cost and while checking to allow or disallow
parallel inserts in IsParallelInsertInCTASAllowed(), we need to add an
assert something like Assert(cost_ignored_in_cost_gather && allow)
before return allow;

This assertion fails 1) either if we have not ignored the cost but
allowing parallel inserts 2) or we ignored the cost but not allowing
parallel inserts.

1) seems to be fine, we can go ahead and perform parallel inserts. 2)
is the concern that the planner would have wrongly chosen the parallel
plan, but in this case also isn't it better to go ahead with the
parallel plan instead of failing the query?

+        /*
+         * We allow parallel inserts by the workers only if the Gather node has
+         * no projections to perform and if the upper node is Gather. In case,
+         * the Gather node has projections, which is possible if there are any
+         * subplans in the query, the workers can not do those projections. And
+         * when the upper node is GatherMerge, then the leader has to perform
+         * the final phase i.e. merge the results by workers.
+         */
+        allow = ps && IsA(ps, GatherState) && !ps->ps_ProjInfo &&
+                plannedstmt->parallelModeNeeded &&
+                plannedstmt->planTree &&
+                plannedstmt->planTree->lefttree &&
+                plannedstmt->planTree->lefttree->parallel_aware &&
+                plannedstmt->planTree->lefttree->parallel_safe;
+
+        return allow;
+    }

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