PolarDB:ST_PointN

構文

geometry ST_PointN(geometry aLinestring , integer n);

パラメーター

説明　

• If you specify an object that is not a LineString object, the ST_PointN function returns NULL.
• If the index that you specify is a negative number, this function counts the points forward from the end of the LineString object. Negative values are counted from the end of the LineString object and -1 is the last point.
• If you want to obtain the Nth point of each LineString in a MultiLineStirng object, you must use the ST_PointN function with the ST_Dump function.
• The ST_PointN function supports circular strings and curves.
• The ST_PointN function supports 3D objects and does not delete z coordinates.

例

• Obtain the Nth point of a LineString object by using the default parameter settings.

  SELECT ST_AsText(ST_PointN('LINESTRING(0 0,2 2)'::geometry,1));
   st_astext
  ------------
   POINT(0 0)
  (1 row)
                      

• Obtain the Nth point of a LineString object by using the backward counting order.

  SELECT ST_AsText(ST_PointN('LINESTRING(0 0,2 2)'::geometry,-1));
   st_astext
  ------------
   POINT(2 2)
  (1 row)