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    PolarDB:ST_ConcaveHull

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    PolarDB:ST_ConcaveHull

    Last Updated:Mar 30, 2026

    Returns the concave hull of one or more geometry objects. Think of a concave hull as shrink-wrap around a set of geometries — it fits more tightly than a convex hull, which behaves more like a rubber band stretched around the outermost points.

    Syntax

    geometry ST_ConcaveHull(geometry geomA, float targetPercent, boolean allowHoles)

    Parameters

    Parameter Type Description Default
    geomA geometry The input geometry object. —
    targetPercent float The ratio of the concave hull area to the convex hull area. Lower values produce tighter hulls but take longer to compute. —
    allowHoles boolean Specifies whether to allow holes in the resulting polygon. false

    Usage notes

    Choosing a `targetPercent` value

    targetPercent controls the trade-off between precision and performance:

    • Start with 0.99. In some scenarios, this is as fast as computing the convex hull, and the resulting hull is almost always tighter than 99% of the convex hull area.

    • Decrease from there to find the best fit. Smaller values increase computation time and the risk of topology exceptions.

    • To reduce output precision after computing, apply ST_SimplifyPreserveTopology (slower, always returns valid geometry) or ST_SnapToGrid (faster, but may return invalid geometry).

    Combining with other functions

    ST_ConcaveHull is not an aggregate function. To compute the concave hull over a set of geometries, combine it with:

    • ST_Collect — for points, LineString objects, or GeometryCollection objects

    • ST_Union — for polygons

    Example pattern: ST_ConcaveHull(ST_Collect(somepointfield), 0.80)

    Passing invalid geometry objects to ST_ConcaveHull causes the function to fail.

    Dimension and input behavior

    • The dimension of the returned hull is never higher than the dimension of the input polygon object.

    • ST_ConcaveHull is primarily used with MULTI and GeometryCollection objects.

    • For image recognition and similar spatial analysis tasks, ST_ConcaveHull encloses all input geometries more precisely than a convex hull.

    Examples

    Compare targetPercent values

    The following query compares the original geometry against concave hulls computed at targetPercent = 0.99 and targetPercent = 0.98.

    SELECT g, ST_ConcaveHull(g, 0.99), ST_ConcaveHull(g, 0.98), g
FROM (
    SELECT 'MULTIPOLYGON(((0 0,1 0,1 1,0 1,0 0)),((0 6,6 3,6 6,0 6)))'::geometry AS g
) AS test;

    123

    Compute a concave hull over a set of points

    Use ST_Collect to aggregate point observations before passing them to ST_ConcaveHull.

    -- Estimate the affected area based on point observations, grouped by type
SELECT disease_type,
    ST_ConcaveHull(ST_Collect(obs_point), 0.99) AS geom
FROM disease_obs
GROUP BY disease_type;

    What's next

    • ST_ConvexHull — compute the convex hull of a geometry

    • ST_Collect — aggregate geometries before passing to ST_ConcaveHull

    • ST_Union — merge polygon geometries before computing the hull