ST_CoverageUnion
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Name

ST_CoverageUnion — Computes the union of a set of polygons forming a coverage by removing shared edges.

Synopsis

geometry ST_CoverageUnion(geometry set geom);

Description

An aggregate function which unions a set of polygons forming a polygonal coverage. The result is a polygonal geometry covering the same area as the coverage. This function produces the same result as ST_Union, but uses the coverage structure to compute the union much faster.

[Note]

If the input is not a valid coverage there may be unexpected artifacts in the output (such as unmerged or overlapping polygons). Use ST_CoverageInvalidEdges to determine if a coverage is valid.

Availability: 3.4.0 - requires GEOS >= 3.8.0

Examples

Union a polygonal coverage.

Code
WITH coverage(id, geom) AS (VALUES
  (1, 'POLYGON ((10 10,10 150,80 190,110 150,90 110,40 110,50 60,10 10))'::geometry),
  (2, 'POLYGON ((120 10,10 10,50 60,100 70,120 10))'::geometry),
  (3, 'POLYGON ((140 80,120 10,100 70,40 110,90 110,110 150,140 80))'::geometry),
  (4, 'POLYGON ((140 190,120 170,140 130,160 150,140 190))'::geometry),
  (5, 'POLYGON ((180 160,170 140,140 130,160 150,140 190,180 160))'::geometry)
)
SELECT ST_CoverageUnion(geom) AS wkt
  FROM coverage;
Output
MULTIPOLYGON(((10 150,80 190,110 150,140 80,120 10,10 10,10 150),(50 60,100 70,40 110,50 60)),((120 170,140 190,180 160,170 140,140 130,120 170)))
Figure
Geometry figure for visual-st-coverageunion-01