gistfile1

SELECT

    ST_ConvexHull(ST_Collect(planet_osm_point.way)) AS convexHull,
    ST_Centroid(ST_ConvexHull(ST_Collect(planet_osm_point.way))) AS centroid

FROM

    planet_osm_polygon,
    planet_osm_point

WHERE

    ST_DWithin(ST_Centroid(planet_osm_polygon.way), planet_osm_point.way, 2000)

AND

    planet_osm_polygon.name = 'Erika-Hess-Eisstadion'

AND

    planet_osm_point.amenity = 'restaurant';