[postgis-devel] [SoC] GSoC 2021 - Week 3 Report - Implement pre-sorting methods before GiST index building

[Darafei "Komяpa" Praliaskouski]

Hi,


On Sun, Jun 27, 2021 at 5:55 PM Han Wang <hanwgeek at gmail.com> wrote:

> Hi all,
>
> I am here to share with you my Week3 report. You can also find it at [1]
>
> *Coding Phase *:
>
>    - Refactor the code structure, moving the hash function to the
>    inline module
>    - Create a pull request and receive suggestions from the community and
>    mentors[2]
>    - Finish the morton hash function
>
> I used `interleave` to implement a Morton(z-order) hash. What's more, I
> try to return the `x` of a `BOX2DF` as the hash order of the geometry
> object.
> What is confusing is that the Hilbert hash, the morton hash and the simple
> `x` show little difference in the performance test. So I think it is
> necessary to augment our test dataset and use some read world data.
>


 - Try something broken from the start, like hash(x). Does it become worse?
If not, something is terribly wrong in benchmark code.

 - To check the ordering of the index, you can create an index and then
CLUSTER the table using that. Then ST_MakeLine(geom order by ctid) will
create a line that follows the objects in the order they're in the index.

 - I hope you're using at least a couple millions of objects. If not, take
any OpenStreetMap country (I use Belarus usually), get it to the db
(osmium-tool and its osmium export -f pg is good) and index that.




>
> *Plans for next week*:
>
>    - Create larger random data or use real-world data for testing
>    - Evaluate the stability and efficiency of hash functions
>    - Searching for a more efficient hash function
>
>
> If you have any questions or suggestions, please let me know.
>
> [1]
> https://trac.osgeo.org/postgis/wiki/ImplementSortingMethodsBeforeGistIndexBuilding
> [2] https://github.com/postgis/postgis/pull/619
>
> Best regards,
> Han
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>


-- 
Darafei "Komяpa" Praliaskouski
OSM BY Team - http://openstreetmap.by/
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