ST_PointN
ST_PointN returns a point in a linestring as specified by an index value. Negative index values are counted backward from the end of the linestring, so that 1 is the last point.
The dimension of the returned geometry is the same as that of the input geometry.
Syntax
ST_PointN(geom, index)
Arguments
 geom

A value of data type
GEOMETRY
or an expression that evaluates to aGEOMETRY
type. The subtype must beLINESTRING
.  index

A value of data type
INTEGER
that represents the index of a point in a linestring.
Return type
GEOMETRY
of subtype POINT
.
The spatial reference system identifier (SRID) value of the returned geometry is set to 0.
If geom or index is null, then null is returned.
If index is out of range, then null is returned.
If geom is empty, then null is returned.
If geom is not a LINESTRING
, then null is returned.
Examples
The following SQL returns an extended wellknown text (EWKT) representation of a sixpoint
LINESTRING
to a GEOMETRY
object and returns the point at index 5 of the linestring.
SELECT ST_AsEWKT(ST_PointN(ST_GeomFromText('LINESTRING(0 0,10 0,10 10,5 5,0 5,0 0)',4326), 5));
st_asewkt

SRID=4326;POINT(0 5)