Use ST_Perimeter to calculate the perimeter of geometry and geography objects - ApsaraDB RDS

Calculates the 2D perimeter of a geometry or geography object.

Syntax

float ST_Perimeter(geometry g1);
float ST_Perimeter(geography geog, boolean useSpheroid);

Parameter	Description
`g1`	The geometry object.
`geog`	The geography object.
`useSpheroid`	Specifies whether to use an ellipsoid.

For ST_Surface and ST_MultiSurface objects (polygons and MultiPolygons), the function returns the 2D perimeter.
For non-areal objects (points, lines), the function returns 0. To measure the length of a LineString or MultiLineString, use ST_Length instead.
For geometry input, the result is in the unit defined by the object's spatial reference system.
For geography input, the result is an inverse geodesic distance measured in meters.

SELECT ST_Perimeter('POLYGON((0 0,0 2,2 2,2 0,0 0))'::geometry);
 st_perimeter
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            8
(1 row)