Add ST_Split

[yharby]

ST_Split (split a geometry by a blade geometry, returning the pieces as a GEOMETRYCOLLECTION) is a common PostGIS function missing from duckdb-spatial.

Important, the bundled GEOS is 3.14.1, and native GEOSSplit only landed in GEOS 3.15, so a native bind is not linkable today. The PostGIS recipe composes the result entirely from primitives duckdb-spatial already ships, so it works on 3.14.1 now, and a native GEOSSplit bind can replace it later behind a version guard once the vcpkg baseline reaches 3.15.

Polygon by line recipe (mirrors PostGIS lwgeom_split in lwgeom_geos_split.c)

flowchart TD
    A[target polygon] --> B[ST_Boundary]
    C[blade line] --> D[ST_Union of boundary and blade]
    B --> D
    D --> E[ST_Polygonize rebuild faces]
    E --> F[keep faces whose point on surface lies inside the original]
    F --> G[GEOMETRYCOLLECTION of pieces]

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Line by point or line recipe, ST_Node of the union then collect the parts, or ST_Difference for simple cases.

Implementer tips

• Every primitive above already exists as a registered function and a GeosGeometry wrapper (ST_Boundary, ST_Union, ST_Polygonize, ST_PointOnSurface, ST_Contains, ST_Difference, ST_Node), so a composed ST_Split chains existing wrappers in one Execute with no new GEOS calls.
• Return a GEOMETRYCOLLECTION of the pieces, matching PostGIS.
• Path B, a thin GEOSSplit_r bind behind #if (GEOS_VERSION_MAJOR > 3) || (GEOS_VERSION_MAJOR == 3 && GEOS_VERSION_MINOR >= 15), switched on when the baseline advances.

This is adjacent to the ST_Subdivide work in #719 and #583. Both partition geometries, though split by a blade is distinct, so a shared note on scope may help.

Sharing this as research in case it helps whoever picks it up.