Hi Nuno,
not against but I have a couple of worries.

1) The where clause generated by GeoServer talks about attributes visible
when the full query is run, depending on the query
complexity one might decide to place the where somewhere that only has a
subset of such attributes computed (e..g, case with inner queries).
This will eventually break the query if the wrong attribute is filtered
upon. I guess this migth be dealt with at the documentation level:
"hey dude, we give you this opportunity, the mechanism works like this
<lenghly explanation here>, you can shoot yourself in the foot with it,
don't blame us"

2) The SQL View mechanism is generic, cross database... will whatever query
location work along with paging and query hints across
    all databases? What I'm asking here is to consider all JDBCDataStore
implementation, as this is not "Oracle sql views", but "sql views" :-p
    Again, this might be documented, e.g., "Sorry XYZ users, :where_clause:
is not for you"

Cheers
Andera


On Wed, Mar 7, 2018 at 3:59 PM, Nuno Oliveira <
nuno.olive...@geo-solutions.it> wrote:

> Hi all,
>
> Sorry for the cross porting but this touches the two projects ...
>
> I would like to extend the current support of SQL views (Virtual Tables)
> to
> allow us to add a placeholder for the where clause where GeoServer will
> add the needed filter.
>
> This comes handy (better performance) when we have extra operations that
> can  be done on top of the rows filtered with the GeoServer filter first.
>
> Consider the following use case: we have several meteorological stations
> that have several measurements (wind speed, temperature, humidity,
> etc ..) and for each one we are only interested in the last measurement for
> each available measurement type.
>
> We can create a query that returns exactly what we want:
>
> SELECT STATION_NAME,
>        MEASUREMENT,
>        MEASUREMENT_TYPE,
>        LOCATION
> FROM
>   (SELECT st.common_name AS STATION_NAME,
>           ob.value AS MEASUREMENT,
>           pr.param_name AS MEASUREMENT_TYPE,
>           st.position AS LOCATION,
>           *ROW_NUMBER() OVER(PARTITION BY st.id <http://st.id>,
> pr.param_name*
> *                            ORDER BY ob.time DESC) AS RANK*
>    FROM meteo.meteo_stations st
>    LEFT JOIN meteo.meteo_observations ob ON st.id = ob.station_id
>    LEFT JOIN meteo.meteo_parameters pr ON ob.parameter_id = pr.id) AS
> stations
> WHERE RANK = 1;
>
> The issue is that this query will NOT be as efficient as it can be when
> used with GeoServer,
> actually the performance will be quite bad. This happens because GeoServer
> will add
> is WHERE clause around the query above, which means that the ranking will
> happen
> for all the data set first and only after we will filter what we don't
> need.
>
> The following query that uses the :where_clause: placeholder will do the
> opposite, we will first
> filter what we don't want and then do the ranking:
>
> SELECT STATION_NAME,
>        MEASUREMENT,
>        MEASUREMENT_TYPE,
>        LOCATION
> FROM
>   (SELECT STATION_NAME,
>           MEASUREMENT,
>           MEASUREMENT_TYPE,
>           LOCATION,
>           *ROW_NUMBER() OVER(PARTITION BY STATION_ID, MEASUREMENT_TYPE*
> *                            ORDER BY TIME DESC) AS RANK*
>    FROM
>      (SELECT st.id AS STATION_ID,
>              st.common_name AS STATION_NAME,
>              ob.value AS MEASUREMENT,
>              pr.param_name AS MEASUREMENT_TYPE,
>              ob.time AS TIME,
>              st.position AS LOCATION
>       FROM meteo.meteo_stations st
>       LEFT JOIN meteo.meteo_observations ob ON st.id = ob.station_id
>       LEFT JOIN meteo.meteo_parameters pr ON ob.parameter_id = pr.id
>       WHERE 1 = 1 *:where_clause:*) AS stations_filtered) AS stations
> WHERE RANK = 1;
>
> This will make the usage of some SQL analytic functions affordable
> (performance wise) with GeoServer.
>
> The changes in the code are quite small (a few lines), and this is an
> additive change fully backward compatible.
>
> Any objection ? comments ?
>
> Regards,
>
> Nuno Oliveira
>
> --
> 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 di Montramito 3/A
> 55054  Massarosa (LU)
> Italy
> phone: +39 0584 962313 <+39%200584%20962313>
> fax:      +39 0584 1660272 <+39%200584%20166%200272>
> http://www.geo-solutions.ithttp://twitter.com/geosolutions_it
>
> -------------------------------------------------------
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-- 

Regards,

Andrea Aime

==
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 di Montramito 3/A
55054  Massarosa (LU)
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
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loro utilizzo è consentito esclusivamente al destinatario del messaggio,
per le finalità indicate nel messaggio stesso. Qualora riceviate questo
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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