On Wed, Oct 14, 2020 at 2:46 PM Bharath Rupireddy <bharath.rupireddyforpostg...@gmail.com> wrote: > > On Tue, Oct 6, 2020 at 10:58 AM Amit Kapila <amit.kapil...@gmail.com> wrote: > > > > > > While skimming through the patch, a small thing I noticed: > > + /* > > + * SELECT part of the CTAS is parallelizable, so we can make > > + * each parallel worker insert the tuples that are resulted > > + * in it's execution into the target table. > > + */ > > + if (!is_matview && > > + IsA(plan->planTree, Gather)) > > + ((DR_intorel *) dest)->is_parallel = true; > > + > > > > I am not sure at this stage if this is the best way to make CTAS as > > parallel but if so, then probably you can expand the comments a bit to > > say why you consider only Gather node (and that too when it is the > > top-most node) and why not another parallel node like GatherMerge? > > > > If somebody expects to preserve the order of the tuples that are > coming from GatherMerge node of the select part in CTAS or SELECT INTO > while inserting, now if parallelism is allowed, that may not be the > case i.e. the order of insertion of tuples may vary. I'm not quite > sure, if someone wants to use order by in the select parts of CTAS or > SELECT INTO in a real world use case. Thoughts? >
I think there is no reason why one can't use ORDER BY in the statements we are talking about here. But, I think we can't enable parallelism for GatherMerge is because for that node we always need to fetch the data in the leader backend to perform the final merge phase. So, I was expecting a small comment saying something on those lines. > > > > > Need to analyze whether to allow parallelism if CTAS has prepared > > statements or with no data. > > > > For prepared statements, the parallelism will not be picked and so is > parallel insertion. > Hmm, I am not sure what makes you say this statement. The parallelism is enabled for prepared statements since commit 57a6a72b6b. > > I'm listing the things that are still pending. > > 1. How to represent the parallel insert for CTAS in explain plans? The > explain CTAS shows the plan for only the SELECT part. How about having > some textual info along with the Gather node? I'm not quite sure on > this point, any suggestions are welcome. > I am also not sure about this point because we don't display anything for the DDL part in explain. Can you propose by showing some example of what you have in mind? -- With Regards, Amit Kapila.
