ST_Union behaviour

[Regina Obe]

Are you unioning one geometry or many?

 

The only reason I can think of why ST_Union would return unchanged overlapping polygons is if you fed it a geometry collection or invalid multipolygon with overlapping polygons.

 

In these cases you should be using ST_UnaryUnion  https://postgis.net/docs/ST_UnaryUnion.html

 

 

 

From: Antonio Valanzano <anvalanz at gmail.com> 
Sent: Monday, December 23, 2024 3:47 AM
To: postgis-users at lists.osgeo.org
Subject: ST_Union behaviour

 

Does someone know why ST_Union behaves differently with linestrings and polygons?

 

If a table contains linestrings that overlap at some points then ST_Union creates a collection of linestrings  that are splitted at intersections.

 

If a table contains polygons with overlaps then ST_Union creates a collection of polygons that are still overlapped and are not splitted (the result contains only the original polygons).

 

Thanks in advance.

 

Antonio

 

 

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