PostGIS Connection In QGIS. Vector Query Results of GeometryCollection Load Layer as Raster

Question

https://postgis.net/docs/ST_VoronoiPolygons.html

I am trying to load this query result using PostGIS 2.5 connection in QGIS 3.4, QGIS 2.18.

SELECT ST_VoronoiPolygons(geom) As geom FROM (SELECT 'MULTIPOINT (50 30, 60 30, 100 100,10 150, 110 120)'::geometry As geom ) As g;

Getting the error of the vector layer GeometryCollection result as Raster Layer in QGIS.

Results =>

I have tried these PostGIS functions to verify results,

1. https://postgis.net/docs/ST_IsValid.html
2. https://postgis.net/docs/ST_MakeValid.html
3. https://postgis.net/docs/ST_IsValidReason.html

I have also applied ST_AsGeoJSON and ST_AsText, results is GeometryCollection, and valid.

I have imported the GeoJSON in https://mapshaper.org/, it's okay.

After extracting/dumping the Features from GeometryCollection with following functions it's working fine.

1. ST_CollectionExtract with Polygon selection.
2. ST_Dump.

Why QGIS Load GeometryCollection vector query result as a raster layer? Is there any method available under QGIS to load GeometryCollection?

Answer 1

Qgis does not like GeometryCollection, you should convert geometry collection result of ST_VoronoiPolygons to MultiPolygon using ST_CollectionExtract. For example if you have a point layer to generate voronoi cells:

with p as (
    select ST_Union(geom) as geom from you_point_layer
)
select 1 as id, 
ST_CollectionExtract(ST_VoronoiPolygons(geom), 3) as geom from p;

The second parameter of ST_CollectionExtract is the geometry type to extract (3 = polygon). The result is one multipolygon geometry. Next picture shows how to use it in DB Manager for a point layer to get voronoi cells (varos is my point layer)

And loading it into QGIS: