Re: Questions about support function and abbreviate

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От Han Wang
Тема Re: Questions about support function and abbreviate
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Msg-id CADCmNp1dYmPq9rcy5ujx+3zH51d7uCX5iR+5jAJfyhr4pe97BA@mail.gmail.com
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Ответ на Re: Questions about support function and abbreviate  (Darafei "Komяpa" Praliaskouski <me@komzpa.net>)
Ответы Re: Questions about support function and abbreviate  (Giuseppe Broccolo <g.broccolo.7@gmail.com>)
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Hi Darafei,

Thanks for your reply.

However, I still don't get the full picture of this.  Let me make my question more clear.

First of all, in the `gistproc.c` of Postgres, it shows that the `abbreviate` attributes should be set before the `abbrev_converter` defined. So I would like to know where to define a `SortSupport` structure with `abbreviate` is `true`.  

Secondly, in the support functions of internal data type `Point`, the `abbrev_full_copmarator` just z-order hash the point first like the `abbrev_converter` doing and then compare the hash value. So I don't know the difference between `full_comparator` and `comparator` after `abbrev_converter`.

Best regards,
Han

On Sat, Jun 12, 2021 at 2:55 PM Darafei "Komяpa" Praliaskouski <me@komzpa.net> wrote:
Hello,

the abbrev_converter is applied whenever it is defined. The values are sorted using the abbreviated comparator first using the shortened version, and if there is a tie the system asks the real full comparator to resolve it. 

This article seems to be rather comprehensive: https://brandur.org/sortsupport

On Sat, Jun 12, 2021 at 9:51 AM Han Wang <hanwgeek@gmail.com> wrote:
Hi all,

I am trying to implement a sort support function for geometry data types in PostGIS with the new feature `SortSupport`. However, I have a question about this.

I think it is hardly to apply a sort support function to a complex data type without the `abbrev_converter` to simply the data structure into a single `Datum`. However, I do not know how the system determines when to apply the converter.

I appreciate any answers or suggestions. I am looking forward to hearing from you.

Best regards,
Han


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

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