ST_Centroid(geometry);
geometry ：a specified ST_Geometry
e.g.：
select ST_AsText(ST_Centroid('0103000020E610000001000000050000000EBF9B6ED9775E40B96DDFA3FE723C4019E6046D72775E400CAD4ECE50743C40ED832C0B26785E40B6D9588979763C40A471A8DF85785E409605137F14753C400EBF9B6ED9775E40B96DDFA3FE723C40'));
结果：
POINT(121.874850835107 28.4559363140394)
ST_Centroid(ST_GeomFromText(text WKT));
WKT: Well-Known Text representation
e.g.：select ST_AsText(ST_Centroid(ST_GeomFromText('POLYGON((121.872646 28.449198,121.866359 28.454358,121.877322 28.462792,121.883171 28.457344,121.872646 28.449198))')));
结果：
POINT(121.874850835107 28.4559363140394)