Name
ST_LongestLine — Returns the 2D longest line between two geometries.
Synopsis
geometry ST_LongestLine(geometry
g1, geometry
g2);
Description
Returns the 2-D longest line between the points of two geometries.
The function returns the first longest line if more than one is found. The line returned starts on g1 and ends on g2. The length of the line is equal to the distance returned by ST_MaxDistance.
Availability: 1.5.0
Examples
 Longest line between point and line
SELECT ST_AsText(
ST_LongestLine('POINT(100 100)'::geometry,
'LINESTRING (20 80, 98 190, 110 180, 50 75 )'::geometry)
) As lline;
lline
-----------------
LINESTRING(100 100,98 190)
|  longest line between polygon and polygon
SELECT ST_AsText(
ST_LongestLine(
ST_GeomFromText('POLYGON((175 150, 20 40,
50 60, 125 100, 175 150))'),
ST_Buffer(ST_GeomFromText('POINT(110 170)'), 20)
)
) As llinewkt;
lline
-----------------
LINESTRING(20 40,121.111404660392 186.629392246051)
|
 longest straight distance to travel from one part of an elegant city to the other Note the max distance = to the length of the line.
SELECT ST_AsText(ST_LongestLine(c.the_geom, c.the_geom)) As llinewkt,
ST_MaxDistance(c.the_geom,c.the_geom) As max_dist,
ST_Length(ST_LongestLine(c.the_geom, c.the_geom)) As lenll
FROM (SELECT ST_BuildArea(ST_Collect(the_geom)) As the_geom
FROM (SELECT ST_Translate(ST_SnapToGrid(ST_Buffer(ST_Point(50 ,generate_series(50,190, 50)
),40, 'quad_segs=2'),1), x, 0) As the_geom
FROM generate_series(1,100,50) As x) AS foo
) As c;
llinewkt | max_dist | lenll
---------------------------+------------------+------------------
LINESTRING(23 22,129 178) | 188.605408193933 | 188.605408193933
|