Re: GSoC 2015: SP-GIST for geometrical objects

On Mar 27, 2015 6:41 PM, "Dima Ivanovskiy" <dima-iv(at)mail(dot)ru> wrote:
>
>
>> On Mar 27, 2015 11:08 AM, "Dima Ivanovskiy" <dima-iv(at)mail(dot)ru> wrote:
>> >
>> > Hello, I am Dmitrii, student of Moscow Institute of Physics and
Technology
>> >
>> > Abstract:
>> >
>> > I chose project "Indexing prolonged geometrical objects (i.e. boxes,
circles, polygons, not points) with SP-GiST by mapping to 4d-space".
>> > According to the presentation
>> > https://www.pgcon.org/2011/schedule/attachments/197_pgcon-2011.pdf
>> > SP-GIST 3 times faster than GiST in some cases. But GIST supports
geometrical data types:
>> > box, circle, polygon with operators: && &> &< &<| >> << <<| <@ @> @
|&> |>> ~ ~=
>> > Popular spatial extension PostGIS doesn't include SP-GIST, but has a
lot of geometrical features.
>> >
>> > Project details:
>> >
>> > After meeting with Alexander Korotkov, I wrote some plan.
>> > Using of K-D-tree and Quadtree in building index for geometrical data
types can increase speed of search in some cases.
>> > The main idea is representing 2-D geometrical objects in their
bounding box. Set of 2-D boxes is 4-D space.
>> > New _ops will work with points from 4-D space, for example kd_box_ops,
quad_circle_ops and will support all geometrical operators.
>> > After conversion object to their bounding box algo has set of tuples
(x1, y1, x2, y2).
>> > Our goal is separate this space the most equally. If we talk about
K-D-tree, on first step K-D-tree algorithm will split space in 2 parts by
the first coordinate, in next step by the second coordinate etc., after
4-th coordinate we repeat this procedure.
>> > At the end we have index at geometrical objects and use traversal tree
for every search operator.
>> >
>> > Postgresql has already has realization ideas of MBR in
gist/gistproc.c. So I will transfer this realization to other type of tree.
>> >
>> > Of cource, I assume that SP-GIST can be not the best decision of this
problem. So after testing this clear methods, I will try to find more
effective way. Maybe with using combination of different spatial tree
structures.
>> >
>> > Project Schedule:
>> >
>> > until May 25
>> >
>> > Read documentation and source code, clarify details of implementation.
>> >
>> > 1st month
>> >
>> > Implement new '_ops' with all geometrical operators for box, circle,
polygon
>> >
>> > 2nd month
>> >
>> > Research new methods for increase speed of geometrical query
>> >
>> > 3rd month
>> >
>> > Final refactoring, testing and submitting a patch.
>> >
>> >
>> > Links:
>> >
>> > http://www.sai.msu.su/~megera/postgres/talks/gist_tutorial.html -
about GIST
>> > https://toster.ru/q/27135#answer_110197 - people need SP-GIST for cubes
>> > http://www.slideshare.net/profyclub_ru/o-lt - presentation about
indexes
>> > http://pgconf.ru/static/presentations/2015/korotkov_spatial.pdf -
working with geo objects
>> >
>> Nice proposal.
>>
>> Dynamic Kdtrees can perform badly as the splitting median can get way
off as updates are coming. What are your thoughts about that?
>>
>> Also what's up with the 4d space? I don't quite get it. These types are
2 or 3 dimensions.
>
>
> I read spgist README one more time. I didn't find the mechanism for
maintaining good balance after updates.
> I think we can use Bkd-Tree,
https://www.cs.duke.edu/~pankaj/publications/papers/bkd-sstd.pdf. But It
can be not the best solving.
> I include Research time in 2nd month of timeline.
>
> About 4d space. All these types are 2 dimensional.
> Just as in R-tree object is approximated by MBR. MBR for 2d-objects can
be mapped to 4d-point. More general, nd-object MBR can be mapped into
2nd-point.
>
>

The reason I said that is because you pointed performance as one
motivation factor (and the lack of balancing properties can degrade
performance really fast on larger indexes).

The Bkd variant seems interesting but I don't think spgist provides enough
abstraction to implement it.

A bounding box can still be inserted/queried with a 2d kdtree so I don't
know why you call it 4d. I assume it's a matter of naming.

Overall the proposal seems totally doable, although I have doubts about its
usefulness. But I'm no one... You should have some feedback from the
mentors soon.