On Thu, Sep 14, 2017 at 8:30 PM, Amit Khandekar <amitdkhan...@gmail.com> wrote: > On 11 September 2017 at 18:55, Amit Kapila <amit.kapil...@gmail.com> wrote: >>> >> >> How? See, if you have four partial subpaths and two non-partial >> subpaths, then for parallel-aware append it considers all six paths in >> parallel path whereas for non-parallel-aware append it will consider >> just four paths and that too with sub-optimal strategy. Can you >> please try to give me some example so that it will be clear. > > Suppose 4 appendrel children have costs for their cheapest partial (p) > and non-partial paths (np) as shown below : > > p1=5000 np1=100 > p2=200 np2=1000 > p3=80 np3=2000 > p4=3000 np4=50 > > Here, following two Append paths will be generated : > > 1. a parallel-aware Append path with subpaths : > np1, p2, p3, np4 > > 2. Partial (i.e. non-parallel-aware) Append path with all partial subpaths: > p1,p2,p3,p4 > > Now, one thing we can do above is : Make the path#2 parallel-aware as > well; so both Append paths would be parallel-aware. >
Yes, we can do that and that is what I think is probably better. So, the question remains that in which case non-parallel-aware partial append will be required? Basically, it is not clear to me why after having parallel-aware partial append we need non-parallel-aware version? Are you keeping it for the sake of backward-compatibility or something like for cases if someone has disabled parallel append with the help of new guc in this patch? > > So above, what I am saying is, we can't tell which of the paths #1 and > #2 are cheaper until we calculate total cost. I didn't understand what > did you mean by "non-parallel-aware append will consider only the > partial subpaths and that too with sub-optimal strategy" in the above > example. I guess, you were considering a different scenario than the > above one. > Yes, something different, but I think you can ignore that as we can discuss the guts of my point based on the example given by you above. -- With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
