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Advanced Queries

Steven
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Within the Advanced Queries section, you can create new queries by executing geospatial SQL on any dataset within your project. Queries can be saved in order to run again at any time, and query results can be saved as a new dataset inside the project. More information about the geospatial functions supported for vector operations can be found at http://postgis.net/docs/manual-2.0/reference.html

One useful example of an advanced geospatial function, performed for a client, entailed the following: A polygon shapefile was generated from an Esri geoprocessing script. In these cases, all the polygons are combined into a single MultiPolygon that occupies a single row. This screencapture depicts the result of a search query, using the Advanced Queries module, on the dataset:

 
When this occurs, and very complex MultiPolygons are generated, processing and rendering the dataset can potentially be very slow and may even fail on some devices.
 
In these cases, we recommend separating very large MultiPolygons back into individual features using PostGIS's ST_Dump function. This is called "explode" in Esri parlance. Here is the 'explode' function (simply replace "dataset_1193" with your dataset):
 
SELECT name, (st_dump(wkb_geometry)).geom
FROM dataset_1193
 
(The parentheses around the 'st_dump' function are necessary because 'st_dump' returns a set structure and we are interested in the geometry portion.)
 
Now the server and mobile devices will be much faster at processing and rendering the data.

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