ST_Centroid
ST_Centroid returns a point that represents a centroid of a geometry as follows:
For
POINT
geometries, it returns the point whose coordinates are the average of the coordinates of the points in the geometry.For
LINESTRING
geometries, it returns the point whose coordinates are the weighted average of the midpoints of the segments of the geometry, where the weights are the lengths of the segments of the geometry.For
POLYGON
geometries, it returns the point whose coordinates are the weighted average of the centroids of a triangulation of the areal geometry where the weights are the areas of the triangles in the triangulation.For geometry collections, it returns the weighted average of the centroids of the geometries of maximum topological dimension in the geometry collection.
Syntax
ST_Centroid(geom)
Arguments
 geom

A value of data type
GEOMETRY
or an expression that evaluates to aGEOMETRY
type.
Return type
GEOMETRY
If geom is null, then null is returned.
If geom is empty, then null is returned.
Examples
The following SQL returns central point of an input linestring.
SELECT ST_AsEWKT(ST_Centroid(ST_GeomFromText('LINESTRING(110 40, 2 3, 10 80, 7 9, 22 33)', 4326)))
st_asewkt

SRID=4326;POINT(15.6965103455214 27.0206782881905)