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

On Sun, Dec 27, 2020 at 2:20 PM Bharath Rupireddy
<bharath.rupireddyforpostgres@gmail.com> wrote:
>
> On Sat, Dec 26, 2020 at 11:11 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> > I have reviewed part of v15-0001 patch, I have a few comments, I will
> > continue to review this.
>
> Thanks a lot.
>
> > 1.
> > Why is this temporary hack? and what is the plan for removing this hack?
>
> The changes in xact.c, xact.h and heapam.c are common to all the
> parallel insert patches - COPY, INSERT INTO SELECT. That was the
> initial comment, I forgot to keep it in sync with the other patches.
> Now, I used the comment from INSERT INTO SELECT patch. IIRC, the plan
> was to have these code in all the parallel inserts patch, whichever
> gets to review and commit first, others will update their patches
> accordingly.
>
> > 2.
> > +/*
> > + * ChooseParallelInsertsInCTAS --- determine whether or not parallel
> > + * insertion is possible, if yes set the parallel insert state i.e. push down
> > + * the dest receiver to the Gather nodes.
> > + */
> > +void ChooseParallelInsertsInCTAS(IntoClause *into, QueryDesc *queryDesc)
> > +{
> > +    if (!IS_CTAS(into))
> > +        return;
> >
> > When will this hit?  The functtion name suggest that it is from CTAS
> > but now you have a check that if it is
> > not for CTAS then return,  can you add the comment that when do you
> > expect this case?
>
> Yes it will hit for explain cases, but I choose to remove this and
> check outside in the explain something like:
> if (into)
>     ChooseParallelInsertsInCTAS()
>
> > Also the function name should start in a new line
> > i.e
> > void
> > ChooseParallelInsertsInCTAS(IntoClause *into, QueryDesc *queryDesc)
>
> Ah, missed that. Modified now.
>
> > 3.
> > +/*
> > + * ChooseParallelInsertsInCTAS --- determine whether or not parallel
> > + * insertion is possible, if yes set the parallel insert state i.e. push down
> > + * the dest receiver to the Gather nodes.
> > + */
> >
> > Push down to the Gather nodes?  I think the right statement will be
> > push down below the Gather node.
>
> Modified.
>
> > 4.
> > intorel_startup(DestReceiver *self, int operation, TupleDesc typeinfo)
> > {
> >      DR_intorel *myState = (DR_intorel *) self;
> >
> >     -- Comment ->in parallel worker we don't need to crease dest recv blah blah
> > +    if (myState->is_parallel_worker)
> >     {
> >         --parallel worker handling--
> >         return;
> >     }
> >
> >     --non-parallel worker code stay right there, instead of moving to else
>
> Done.
>
> > 5.
> > +/*
> > + * ChooseParallelInsertsInCTAS --- determine whether or not parallel
> > + * insertion is possible, if yes set the parallel insert state i.e. push down
> > + * the dest receiver to the Gather nodes.
> > + */
> > +void ChooseParallelInsertsInCTAS(IntoClause *into, QueryDesc *queryDesc)
> > +{
> >
> > From function name and comments it appeared that this function will
> > return boolean saying whether
> > Parallel insert should be selected or not.  I think name/comment
> > should be better for this
>
> Yeah that function can still return void because no point in returning
> bool there, since the intention is to see if parallel inserts can be
> performed, if yes, set the state otherwise exit. I changed the
> function name to TryParallelizingInsertsInCTAS(). Let me know your
> suggestions if that doesn't work out.
>
> > 6.
> >         /*
> > +         * For parallelizing inserts in CTAS i.e. making each parallel worker
> > +         * insert the tuples, we must send information such as into clause (for
> > +         * each worker to build separate dest receiver), object id (for each
> > +         * worker to open the created table).
> >
> > Comment is saying we need to pass object id but the code under this
> > comment is not doing so.
>
> Improved the comment.
>
> > 7.
> > +        /*
> > +         * Since there are no rows that are transferred from workers to Gather
> > +         * node, so we set it to 0 to be visible in estimated row count of
> > +         * explain plans.
> > +         */
> > +        queryDesc->planstate->plan->plan_rows = 0;
> >
> > This seems a bit hackies Why it is done after the planning,  I mean
> > plan must know that it is returning a 0 rows?
>
> This exists to show up the estimated row count(in case of EXPLAIN CTAS
> without ANALYZE) in the output. For EXPLAIN ANALYZE CTAS actual tuples
> are shown correctly as 0 because Gather doesn't receive any tuples.
>     if (es->costs)
>     {
>         if (es->format == EXPLAIN_FORMAT_TEXT)
>         {
>             appendStringInfo(es->str, "  (cost=%.2f..%.2f rows=%.0f width=%d)",
>                              plan->startup_cost, plan->total_cost,
>                              plan->plan_rows, plan->plan_width);
>
> Since it's an estimated row count(which may not be always correct), we
> will let the EXPLAIN plan show that and I think we can remove that
> part. Thoughts?
>
> I removed it in v6 patch set.
>
> > 8.
> > +        char *intoclause_space = shm_toc_allocate(pcxt->toc,
> > +                                                  intoclause_len);
> > +        memcpy(intoclause_space, intoclausestr, intoclause_len);
> > +        shm_toc_insert(pcxt->toc, PARALLEL_KEY_INTO_CLAUSE, intoclause_space);
> >
> > One blank line between variable declaration and next code segment,
> > take care at other places as well.
>
> Done.
>
> I'm attaching the v16 patch set. Please note that I added the
> documentation saying that parallel insertions can happen and a sample
> output of the explain to 0003 patch as discussed in [1]. But I didn't
> move the explain output related code to a separate patch because it's
> a small snippet in explain.c. I hope that's okay.
>
> [1] - https://www.postgresql.org/message-id/CAA4eK1JqwXGYoGa1%2B3-f0T50dBGufvKaKQOee_AfFhygZ6QKtA%40mail.gmail.com
>

Thanks for working on this, I will have a look at the updated patches soon.

-- 
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com