ST_LineInterpolatePoint
ST_LineInterpolatePoint returns a point along a line at a fractional distance from the start of the line.
To determine point equality, ST_LineInterpolatePoint operates on the 2D projection of
the input geometry. If the input geometry is empty, a copy of it is returned in the same
dimension as the input. For 3DZ, 3DM, and 4D geometries, the z
or
m
coordinate is the average of the z
or m
coordinates of the segment where the point lies.
Syntax
ST_LineInterpolatePoint(geom, fraction)
Arguments
 geom

A value of data type
GEOMETRY
or an expression that evaluates to aGEOMETRY
type. The subtype isLINESTRING
.  fraction

A value of data type
DOUBLE PRECISION
that represents the position of a point along the linestring for the line. The value is a fraction in the range 0–1, inclusive.
Return type
GEOMETRY
of subtype POINT
.
If geom or fraction is null, then null is returned.
If geom is empty, then the empty point is returned.
The spatial reference system identifier (SRID) value of the returned geometry is the SRID value of the input geometry.
If fraction is out of range, then an error is returned.
If geom is not a linestring, then an error is returned.
Examples
The following SQL returns a point halfway along a linestring.
SELECT ST_AsEWKT(ST_LineInterpolatePoint(ST_GeomFromText('LINESTRING(0 0, 5 5, 7 7, 10 10)'), 0.50));
st_asewkt

POINT(5 5)
The following SQL returns a point 90 percent of the way along a linestring.
SELECT ST_AsEWKT(ST_LineInterpolatePoint(ST_GeomFromText('LINESTRING(0 0, 5 5, 7 7, 10 10)'), 0.90));
st_asewkt

POINT(9 9)