Re: index unique

> On 8 Jun 2021, at 22:50, Thomas Kellerer <shammat@gmx.net> wrote:
>
> Marc Millas schrieb am 03.06.2021 um 22:51:
>> on a table we need a primary key and to get a unique combinaison, we need 3 columns of that table:
>> 1 of type integer,
>> 1 of type text,
>> 1 of type geometry
>>
>
> How do you define the "uniqueness" of the geometry?

That is actually the big question here. Multiple “unique” geometries can specify the same geometry!

A geom as simple as a line from (0,0) - (1,0) can just as easily be specified as (1,0) - (0,0). That’s the simplest
case,and one could argue that the point of origin is different, but the next example would be a triangle starting at
thesame origin but traversed in different directions. It gets harder the more vertices a polygon has. 

I would argue that a geometry type is ill-suited as a primary key column candidate.

Now, of course, the OP could have a case where their geometries are guaranteed to be unique regardless, but they’d
bettermake sure before adding them to the PK. 

Alban Hertroys
--
If you can't see the forest for the trees,
cut the trees and you'll find there is no forest.