ST_IsRing Returns TRUE if this LINESTRING is both closed and simple. boolean ST_IsRing geometry g Returns TRUE if this LINESTRING is both (ST_StartPoint(g) ~= ST_Endpoint(g)) and (does not self intersect). This method implements the OpenGIS Simple Features Implementation Specification for SQL. OGC SFSQL 1.1 - 2.1.5.1 This method implements the SQL/MM specification: SQL-MM 3: 7.1.6 SQL-MM defines the result of ST_IsRing(NULL) to be 0, while PostGIS returns NULL. This function supports 3d and will not drop the z-index. , ,