Re: How to retain lesser paths at add_path()?

2019年10月8日(火) 1:56 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
>
> Kohei KaiGai <kaigai(at)heterodb(dot)com> writes:
> > 2019年10月7日(月) 23:44 Robert Haas <robertmhaas(at)gmail(dot)com>:
> >> But if we want to stick with the ad-hoc method, we could also just
> >> have four possible return values: dominates, dominated-by, both, or
> >> neither.
>
> > It seems to me this is a bit different from the purpose of this hook.
> > I never intend to overwrite existing cost-based decision by this hook.
> > The cheapest path at a particular level is the cheapest one regardless
> > of the result of this hook. However, this hook enables to prevent
> > immediate elimination of a particular path that we (extension) want to
> > use it later and may have potentially cheaper cost (e.g; a pair of
> > custom GpuAgg + GpuJoin by reduction of DMA cost).
>
> I do not think this will work for the purpose you wish, for the reason
> Tomas already pointed out: if you don't also modify the accept_new
> decision, there's no guarantee that the path you want will get into
> the relation's path list in the first place.
>
Ah, it is right, indeed. We may need to have a variation of add_path()
that guarantee to preserve a path newly added.
We may be utilize the callback to ask extension whether it allows the
new path to be dominated by the existing cheapest path also.

> Another problem with trying to manage this only by preventing removals
> is that it is likely to lead to keeping extra paths and thereby wasting
> planning effort. What if there's more than one path having the property
> you want to keep?
>
My assumption is that upper path tries to walk on the path-list, not only
cheapest one, to construct upper paths with lesser paths if they are capable
for special optimization.
Of course, it is not a cheap way, however, majority of path-nodes are not
interested in the lesser paths, as its sub-path. Only limited number of
extension will walk on the lesser path also?
A separated list is probably an idea, to keep the lesser paths. It is not
referenced at the usual path, however, extension can walk on the list
to lookup another opportunity more than the cheapest path.
In this case, length of the path_list is not changed.

Best regards,
--
HeteroDB, Inc / The PG-Strom Project
KaiGai Kohei <kaigai(at)heterodb(dot)com>