ST_NumPatches

What is ST_NumPatches?

ST_NumPatches is a PostGIS function that returns the number of patches (polygon faces) in a POLYHEDRALSURFACE or TIN geometry.

ST_NumPatches(geometry g) → integer

Returns NULL for geometry types other than POLYHEDRALSURFACE or TIN.

When would you use ST_NumPatches?

Use ST_NumPatches when working with 3D surface models — inspecting mesh complexity, iterating patches in combination with ST_PatchN, or validating imports from 3D formats:

1SELECT model_id, ST_NumPatches(shell_geom) AS face_count
2FROM buildings_3d
3ORDER BY face_count DESC;

Pair with ST_PatchN to access a specific face by 1-based index, analogous to ST_NumGeometries + ST_GeometryN on multi-geometries.

FAQs

What is a patch?

A polygonal face in a 3D surface structure. POLYHEDRALSURFACE is a collection of arbitrary polygon patches; TIN is a triangulated surface where each patch is a triangle.

Does ST_NumPatches work on GeometryCollection?

No — only POLYHEDRALSURFACE and TIN. Use ST_NumGeometries on general collections.

How do I iterate patches?

SELECT ST_PatchN(g, n) FROM generate_series(1, ST_NumPatches(g)) n; or the more idiomatic ST_Dump, which also works on polyhedral surfaces in recent PostGIS.

Is this commonly used?

Not in 2D workflows. Only relevant if you are importing 3D meshes, CAD exports, or BIM data.