ST_ForceCollection
What is ST_ForceCollection?
ST_ForceCollection returns a GEOMETRYCOLLECTION containing the input geometry. A bare POINT becomes GEOMETRYCOLLECTION(POINT(...)), and an existing collection is returned unchanged (though re-emitted as a new value).
ST_ForceCollection(geometry geom) → geometryThe output SRID and vertex data match the input.
When would you use ST_ForceCollection?
Use ST_ForceCollection when a downstream consumer — some GIS engines, serialization formats, or reporting systems — expects every record to be a collection regardless of component count. Forcing a collection avoids type checks and keeps schemas uniform.
It is also useful in CTEs that mix geometry types but feed into a single column: wrapping each row with ST_ForceCollection guarantees a compatible type before further aggregation.
FAQs
Does ST_ForceCollection flatten nested collections?
No. A GeometryCollection passed in is returned as-is. Use ST_CollectionHomogenize to flatten nesting.
How is this different from ST_Multi?
ST_Multi produces a typed MULTI* (MultiPoint, MultiLineString, MultiPolygon), preserving the underlying feature class. ST_ForceCollection always produces a heterogeneous GEOMETRYCOLLECTION, which may be rejected by typed columns such as geometry(MultiPolygon, 4326).
Can I revert with ST_CollectionExtract?
Yes, provided only one geometry type is present. ST_CollectionExtract(geom, 3) pulls polygonal members out and returns a MULTIPOLYGON.
Does ST_ForceCollection change SRID or coordinates?
No. It only changes the outer type wrapper.
