Re: Partition-wise join for join between (declaratively) partitioned tables

On Wed, Mar 15, 2017 at 8:55 AM, Ashutosh Bapat
<ashutosh(dot)bapat(at)enterprisedb(dot)com> wrote:
> Sorry. That was added by my patch to refactor
> set_append_rel_pathlist(). I have added a patch in the series to
> remove that line.

It's not worth an extra commit just to change what isn't broken.
Let's just leave it alone.

>> Very sad. I guess if we had parallel append available, we could maybe
>> dodge this problem, but for now I suppose we're stuck with it.
>
> Really sad. Is there a way to look at the relation (without any
> partial paths yet) and see whether the relation will have partial
> paths or not. Even if we don't have actual partial paths but know that
> there will be at least one added in the future, we will be able to fix
> this problem.

I don't think so. If we know that rel->consider_parallel will end up
true for a plain table, we should always get a parallel sequential
scan path at least, but if there are foreign tables involved, then
nothing is guaranteed.

>> partition_wise_plan_weight may be useful for testing, but I don't
>> think it should be present in the final patch.
>
> partition_join test needs it so that it can work with smaller dataset
> and complete faster. For smaller data sets the partition-wise join
> paths come out to be costlier than other kinds and are never chosen.
> By setting partition_wise_plan_weight I can force partition-wise join
> to be chosen. An alternate solution would be to use
> sample_partition_fraction = 1.0, but then we will never test delayed
> planning for unsampled child-joins. I also think that users will find
> partition_wise_plan_weight useful when estimates based on samples are
> unrealistic. Obviously, in a longer run we should be able to provide
> better estimates.

I still don't like it -- we have no other similar knob.

--
Robert Haas
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