postgis 生成 图形切割

rsql='select r.code,r.name,st_intersection(r.geom, city.geom) as geom ' \
     'from (SELECT * from gis_osm_道路) as r,(select * from gis_县 where 市=\'铜陵市\' and name=\'%s\') as city   ' \
     'where  st_intersects(r.geom, city.geom)'%(town)

其中st_intersection用于相交图形的切割
st_intersects用于相交图形的筛选，用于提高查询速度