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, 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
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Nuno Miguel Carvalho Oliveira
@nmcoliveira
Software Engineer
GeoSolutions S.A.S.
Via di Montramito 3/A
55054 Massarosa (LU)
Italy
phone: +39 0584 962313
fax: +39 0584 1660272
[http://www.geo-solutions.it](http://www.geo-solutions.it)
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