ST_SRID — 返回拓扑几何的空间参考标识符。
integer ST_SRID(topogeometry tg);
Returns the spatial reference identifier of the topology containing the TopoGeometry.
The identifier is recorded for the topology when it is created and refers to an entry in 第 4.5 节 “空间参考系统”.
可用性：3.2.0
Return the SRID of a TopoGeometry stored in the ri_topo topology.
SELECT topology.ST_SRID(topo) AS srid FROM ri.roads WHERE road_name = 'Unknown';
srid ------ 3438