Re: Gather Merge

On Sun, Feb 19, 2017 at 2:22 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Sat, Feb 18, 2017 at 6:43 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> I think there is a value in supporting mark/restore position for any
>> node which produces sorted results, however, if you don't want to
>> support it, then I think we should update above comment in the code to
>> note this fact. Also, you might want to check other places to see if
>> any similar comment updates are required in case you don't want to
>> support mark/restore position for GatherMerge.
>
> I don't think it makes sense to put mark/support restore into Gather
> Merge. Maybe somebody else will come up with a test that shows
> differently, but ISTM that with something like Sort it makes a ton of
> sense to support mark/restore because the Sort node itself can do it
> much more cheaply than would be possible with a separate Materialize
> node. If you added a separate Materialize node, the Sort node would
> be trying to throw away the exact same data that the Materialize node
> was trying to accumulate, which is silly.
>

I am not sure but there might be some cases where adding Materialize
node on top of Sort node could make sense as we try to cost it as well
and add it if it turns out to be cheap.

> Here with Gather Merge
> there is no such thing happening; mark/restore support would require
> totally new code - probably we would end up shoving the same code that
> already exists in Materialize into Gather Merge as well.
>

I have tried to evaluate this based on plans reported by Rushabh and
didn't find any case where GatherMerge can be beneficial by supporting
mark/restore in the plans where it is being used (it is not being used
on the right side of merge join). Now, it is quite possible that it
might be beneficial at higher scale factors or may be planner has
ignored such a plan. However, as we are not clear what kind of
benefits we can get via mark/restore support for GatherMerge, it
doesn't make much sense to take the trouble of implementing it.

>
> A comment update is probably a good idea, though.
>

Agreed.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com