> > create table clip_result as select intersection(poly.wkb_geometry,
> > line.wkb_geometry) as wkb_geometry from poly, line
> 
> create table clip_result as select intersection(poly.wkb_geometry,
> line.wkb_geometry) as wkb_geometry from poly, line where
> overlaps(poly.wkb_geometry,
> line.wkb_geometry)

I would go a step further and add a && bbox check, assuming you have a 
significant number of lines to clip and there is a spatial index on the line 
table:

CREATE TABLE clip_result AS 
SELECT intersection(poly.wkb_geometry, line.wkb_geometry) AS wkb_geometry 
FROM poly, line
WHERE overlaps(poly.wkb_geometry, line.wkb_geometry)
AND poly.wkb_geometry && line.wkb_geometry



