ST_Subdivide

What is ST_Subdivide?

ST_Subdivide takes a geometry and recursively splits it into smaller pieces until each piece has no more than a specified number of vertices. It is a set-returning function — one row per piece — and is the standard tool for making huge geometries (big country polygons, giant coastline multilines) index-friendly and fast to query.

ST_Subdivide(geometry geom, integer max_vertices = 256, float gridSize = -1) → setof geometry

max_vertices defaults to 256 and must be at least 8. The optional gridSize (PostGIS 3.1+) snaps to a fixed precision grid to improve robustness.

When would you use ST_Subdivide?

Use ST_Subdivide when preparing data for fast spatial joins and tiling. Large monster polygons (a country multipolygon, a full continent of coastline) produce oversized GiST index entries that ruin query performance; subdividing them first so each row is small makes ST_Intersects and ST_DWithin dramatically faster. It is also used to produce smaller tiles for vector map rendering, for parallel processing, and for chunked batch overlays.

1-- Precompute a fast-query version of a huge coastline table
2CREATE TABLE coastline_sub AS
3SELECT id, ST_Subdivide(geom, 128) AS geom
4FROM coastline;

FAQs

Why is subdividing faster for spatial joins?

GiST indexes entries are bounding boxes. A huge polygon has a huge bbox that matches most queries, so the index prunes nothing and every candidate must be evaluated by expensive exact-geometry tests. Subdivided pieces have small bboxes, so the index actually helps and queries can run orders of magnitude faster.

What is a good value for max_vertices?

Between 64 and 256 is typical. Too small produces many rows; too large defeats the purpose. Profile on your data — often 128 is a good default.

Does subdivide preserve topology?

The union of all pieces equals the original geometry. Individual pieces are valid polygons/lines sharing edges with their neighbours but no duplicated area. Use ST_Union over an identifier to reassemble the original if needed.

How does ST_Subdivide differ from ST_Split?

ST_Split cuts a geometry with a specific blade you provide. ST_Subdivide automatically splits until each piece is below a vertex threshold, without you specifying cut lines.