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+---
+title: geo: implement ST_3DDWithin and ST_3DDistance
+summary: `st_3ddistance()`
+toc: true
+docs_area: reference.sql
+---
+
+### `st_3ddistance()`
+**Category**: Geospatial Functions
+**Signature**: `st_3ddistance(geometry_a GEOMETRY, geometry_b GEOMETRY) → FLOAT`
+**Description**: Returns the 3-dimensional minimum Cartesian distance between two geometries. If either geometry has no Z component, this is equivalent to `ST_Distance` with Z coordinates treated as 0.
+
+**Example**:
+```sql
+-- Distance between two 3D points
+SELECT ST_3DDistance('POINT Z(0 0 0)'::GEOMETRY, 'POINT Z(1 1 1)'::GEOMETRY);
+-- Returns: 1.7320508075688772
+
+-- Z-only difference
+SELECT ST_3DDistance('POINT Z(0 0 0)'::GEOMETRY, 'POINT Z(0 0 5)'::GEOMETRY);
+-- Returns: 5
+
+-- 2D geometries (Z treated as 0)
+SELECT ST_3DDistance('POINT(0 0)'::GEOMETRY, 'POINT(3 4)'::GEOMETRY);
+-- Returns: 5
+```
+
+### `st_3ddwithin()`
+**Category**: Geospatial Functions
+**Signature**: `st_3ddwithin(geometry_a GEOMETRY, geometry_b GEOMETRY, distance FLOAT) → BOOL`
+**Description**: Returns true if any part of `geometry_a` is within `distance` units of `geometry_b`, using 3D Euclidean distance. This is equivalent to `ST_3DDistance(geometry_a, geometry_b) <= distance`.
+
+**Example**:
+```sql
+-- Check if 3D points are within distance
+SELECT ST_3DDWithin('POINT Z(0 0 0)'::GEOMETRY, 'POINT Z(1 1 1)'::GEOMETRY, 2.0);
+-- Returns: true
+
+-- Check with exact distance threshold
+SELECT ST_3DDWithin('POINT Z(0 0 0)'::GEOMETRY, 'POINT Z(0 0 5)'::GEOMETRY, 5.0);
+-- Returns: true
+
+-- Below threshold
+SELECT ST_3DDWithin('POINT Z(0 0 0)'::GEOMETRY, 'POINT Z(0 0 5)'::GEOMETRY, 4.0);
+-- Returns: false
+```
+
+### `_st_3ddwithin()`
+**Category**: Geospatial Functions
+**Signature**: `_st_3ddwithin(geometry_a GEOMETRY, geometry_b GEOMETRY, distance FLOAT) → BOOL`
+**Description**: Returns true if any part of `geometry_a` is within `distance` units of `geometry_b`, using 3D Euclidean distance. This variant does not utilize any spatial index and is intended for internal use or when spatial indexes should be bypassed.
+
+**Example**:
+```sql
+-- Internal variant without spatial index optimization
+SELECT _ST_3DDWithin('POINT Z(0 0 0)'::GEOMETRY, 'POINT Z(1 1 1)'::GEOMETRY, 2.0);
+-- Returns: true
+```