[GRASS-dev] v.univar vs v.db.univar

[Moritz Lennert]

On 22/06/17 14:21, Markus Metz wrote:
>
>
> On Thu, Jun 22, 2017 at 10:53 AM, Moritz Lennert
> <mlennert at club.worldonline.be <mailto:mlennert at club.worldonline.be>> wrote:
>>
>> On 21/06/17 22:19, Markus Metz wrote:
>>>
>>>
>>> On Wed, Jun 21, 2017 at 4:30 PM, Moritz Lennert
>>> <mlennert at club.worldonline.be <mailto:mlennert at club.worldonline.be>
> <mailto:mlennert at club.worldonline.be
> <mailto:mlennert at club.worldonline.be>>> wrote:
>>>>
>>>>
>>>> On 21/06/17 14:57, Markus Neteler wrote:
>>>>>
>>>>>
>>>>> On Wed, Jun 21, 2017 at 11:54 AM, Moritz Lennert
>>>>> <mlennert at club.worldonline.be <mailto:mlennert at club.worldonline.be>
> <mailto:mlennert at club.worldonline.be <mailto:mlennert at club.worldonline.be>>>
>>>
>>> wrote:
>>>>>>
>>>>>>
>>>>>> Do we really need v.db.univar ? Does it provide anything else/better
>>>
>>> than
>>>>>>
>>>>>> v.univar ?
>>>
>>>
>>> The main difference between v.univar and v.db.univar is that v.univar
>>> works with geometries while v.db.univar works with the attribute table.
>>> If vector geometries share the same category value, or if some vector
>>> geometries have more than one category value, the results of v.univar
>>> and v.db.univar for the same column will be different. If you want
>>> results with category (or class) as unit, use v.db.univar, if you want
>>> results with vector geometry as unit, use v.univar.
>>
>>
>> Right. Didn't think about this. So they are actually complementary.
>>
> [...]
>>
>> However, in the case of multiple cat values per feature, it seems that
> v.univar actually reads the attribute once for every geometry/category
> combination, not only for each feature:
>
> According to the source code, yes.
>
> [...]
>>
>> I don't know if this is the desired feature or if each geometry should
> only be taken into account once, whatever its number of categories. I
> personally would have expected the latter, if we consider v.univar to be
> geometry-based.
>
> If each geometry should be taken into account only once, a corresponding
> category / attribute value would need to be randomly picked. Therefore I
> would prefer to consider all categories for each geometry.

Right.

>>
>> All this definitely needs clearer documentation in the man page.
> Attached an attempt to amend the two respective man pages. MarkusM, is
> this correct ? However, the above question concerning v.univar probably
> should be checked before I commit this.
>
> According to the source code, v.univar reads attributes for all
> categories of a geometry.

Ok.

>
> v.db.univar uses db.univar, not db.select, that's an error in the
> existing manual.

But db.univar uses db.select to access the data, so I don't consider 
this an error in the manual...

>
> I suggest to update the manuals first, then decide if the modules should
> be modified.

Ok. See r71207 and r71208. Please check and tell me if I can backport.

Moritz