[postgis-users] Raster clipping vs. intersection

[Paragon Corporation]

In doing some tests I was somewhat surprised to find out that doing an
ST_Clip operation first before an ST_Intersection operation is significantly
faster than just doing a straight ST_Intersection.

Unfortunately the answers are different and I'm not sure what I am losing.  

I'm guessing with ST_Clip when a pixel only partially intersects a geometry
it is thrown out and with ST_Intersection if a pixel intersects a geometry
it is included.  Is that correct.

This is running in PostGIS 2.1.1 (don't have 2.0 readily set up to compare)


For example:
-- gives an answer of 1258.409 but returns it in 14,431 ms  (without
aggregation returns 307 rows)

SELECT SUM((gval).val* ST_Area((gval).geom))
     / ST_Area(ST_Union((gval).geom)) As avg_elesqm
FROM (
SELECT ST_Intersection(rast,1,buf.geom) As gval
FROM kauai 
     INNER JOIN 
(SELECT ST_Buffer(
      ST_GeomFromText('POINT(444205 2438785)',26904),
        100) As geom
) As buf ON
    ST_Intersects(rast,buf.geom)) As foo;


-- The same operation but adding a ST_Clip step --
For example:
-- gives an answer of 1236.834495 but returns it in 511 ms (without
aggregation returns 281 rows)

SELECT SUM((gval).val* ST_Area((gval).geom))
     / ST_Area(ST_Union((gval).geom)) As avg_elesqm
FROM (
SELECT ST_Intersection(ST_Clip(rast,buf.geom),1,buf.geom) As gval
FROM kauai 
     INNER JOIN 
(SELECT ST_Buffer(
      ST_GeomFromText('POINT(444205 2438785)',26904),
        100) As geom
) As buf ON
    ST_Intersects(rast,buf.geom)) As foo;



Thanks,
Regina