I assume what you want out of a GroupBy + Aggregate function is a list of
results.
Working with these two ideas:
1) FeatureCollection using GroupBy is similar to a series of
FeatureCollections
2) Each of the core visitors should be isolating its state into a result
(this was done so we could go parallel). The results have methods to
combine two answers, and should also have a copy constructor.
With that in mind we should be able to "reset" the result and the end of
each group (using a copy constructor or simply a new result).
The above probably only helps for the ContentDataStore superclass, but that
should get you most of the implementations sorted.
Notes:
- I tend to prefer copy constructors to clonable, we have some trouble with
this when duplicating styles / filters / expressions.
--
Jody Garnett
On 8 February 2016 at 01:21, Andrea Aime <andrea.a...@geo-solutions.it>
wrote:
> Forwarding this one on behalf of Nuno, that is having problems sending
> this mail directly
>
> Cheers
> Andrea
>
> ---------- Forwarded message ----------
> From: Nuno Miguel Carvalho Oliveira <nmco51...@gmail.com>
> Date: Fri, Feb 5, 2016 at 6:44 PM
> Subject: Adding "group by" support to the current aggregation visitors
> To: geotools-devel@lists.sourceforge.net
>
>
> Hi,
>
>
> The current aggregation visitors (AverageVisitor, CountVisitor,
> MaxVisitor, MedianVisitor, MinVisitor, StandardDeviationVisitor and
> SumVisitor) don’t support a “group by” behavior. For example, we are able
> to compute the average electrical consumption per building. But we are not
> able to ask the average electrical consumption per building type.
>
>
> We have came up with three solutions to implement this behavior but none
> of them is perfect.
>
>
> The most straightforward solution would be adding the list of “group by”
> attributes to each current aggregator visitor implementation. But this will
> break backward compatibility with custom store implementations that we are
> not aware of. This solution will also make the current visitors
> unnecessarily more complex, each of them will need to know how to handle
> the “group by” behavior (this can also lead to some code duplication).
>
>
> Another solution is to implement a “group by” visitor that will receive
> an aggregation visitor as a prototype (basically a visitor that will be
> cloned) and a list of “group by” attributes. The current visitors have in
> common the FeatureVisitor interface which is not cloneable. In Java 8 we
> could make the FeatureVisitor extends Cloneable interface and implement a
> default method, but this will not work in GeoServer 2.8.x. So if we extend
> the FeatureVisitor to support Cloneable behavior we are breaking backward
> compatibility. An (awful) solution for this could be implement the
> Cloneable interface in every existing visitor and in the “group by” visitor
> do an “instanceof” checking if the visitor is cloneable. If the visitor is
> not cloneable we could throw an exception or (let's be crazy) do a deep
> serialization copy.
>
>
> Another solution is to implement a “group by” visitor that will receive
> an aggregation visitor name (this would be an Enum), the name of the
> attribute that should be used by the aggregation visitor and a list of
> “group by” attributes. The “group by” visitor will be responsible to
> instantiate the aggregation visitor using the attribute name. The drawback
> of this solution is that the list of supported aggregation visitors will be
> an Enum, which will kill pluggability. So if someone wants to use a new
> aggregation visitor with the “group by” visitor the implementation needs to
> be updated.
>
>
> We will also extend the JDBCDataStore current support for aggregation
> functions to support the "group by" clause.
>
>
> Any feedback on this will be highly appreciated.
>
>
> Regards,
>
>
> Nuno Oliveira
>
> --
> ==
> GeoServer Professional Services from the experts!
> Visit http://goo.gl/it488V for more information.
> ==
> Nuno Miguel Carvalho Oliveira
> @nmcoliveira
> Software Engineer
>
> GeoSolutions S.A.S.
> Via Poggio alle Viti 1187
> 55054 Massarosa (LU)
> Italy
> phone: +39 0584 962313
> fax: +39 0584 1660272
> mob: +39 333 8128928
> http://www.geo-solutions.ithttp://twitter.com/geosolutions_it
>
> -------------------------------------------------------
>
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> --
> ==
> GeoServer Professional Services from the experts! Visit
> http://goo.gl/it488V for more information.
> ==
>
> Ing. Andrea Aime
> @geowolf
> Technical Lead
>
> GeoSolutions S.A.S.
> Via Poggio alle Viti 1187
> 55054 Massarosa (LU)
> Italy
> phone: +39 0584 962313
> fax: +39 0584 1660272
> mob: +39 339 8844549
>
> http://www.geo-solutions.it
> http://twitter.com/geosolutions_it
>
> *AVVERTENZE AI SENSI DEL D.Lgs. 196/2003*
>
> Le informazioni contenute in questo messaggio di posta elettronica e/o
> nel/i file/s allegato/i sono da considerarsi strettamente riservate. Il
> loro utilizzo è consentito esclusivamente al destinatario del messaggio,
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> messaggio senza esserne il destinatario, Vi preghiamo cortesemente di
> darcene notizia via e-mail e di procedere alla distruzione del messaggio
> stesso, cancellandolo dal Vostro sistema. Conservare il messaggio stesso,
> divulgarlo anche in parte, distribuirlo ad altri soggetti, copiarlo, od
> utilizzarlo per finalità diverse, costituisce comportamento contrario ai
> principi dettati dal D.Lgs. 196/2003.
>
>
>
> The information in this message and/or attachments, is intended solely for
> the attention and use of the named addressee(s) and may be confidential or
> proprietary in nature or covered by the provisions of privacy act
> (Legislative Decree June, 30 2003, no.196 - Italy's New Data Protection
> Code).Any use not in accord with its purpose, any disclosure, reproduction,
> copying, distribution, or either dissemination, either whole or partial, is
> strictly forbidden except previous formal approval of the named
> addressee(s). If you are not the intended recipient, please contact
> immediately the sender by telephone, fax or e-mail and delete the
> information in this message that has been received in error. The sender
> does not give any warranty or accept liability as the content, accuracy or
> completeness of sent messages and accepts no responsibility for changes
> made after they were sent or for other risks which arise as a result of
> e-mail transmission, viruses, etc.
>
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