PostGIS intersection teszt

SELECT 'intersection', st_intersection(a.geom,b.geom)
FROM
  (
    SELECT 'a1' id, st_geomfromtext('POLYGON((10 10, 10 100, 50 100, 50 10, 10 10))',23700) geom
    UNION
    SELECT 'a2' id, st_geomfromtext('POLYGON((50 10, 50 100, 100 100, 100 10, 50 10))',23700) geom
  ) a,
  (
    SELECT 'b' id, st_geomfromtext('POLYGON((20 0, 20 50, 80 50, 80 0, 20 0))',23700) geom
  ) b


vs

SELECT a.id, st_union(b.geom) FROM
table1 a
INNER JOIN table2 b ON st_intersects(a.geom, b.geom) GROUP BY a.id)
SELECT g.id, st_union(c.geom, g.geom)
FROM g
INNER JOIN table1 c ON c.id=g.id;