ST_BufferClusteringRequiredDistance

DROP FUNCTION IF EXISTS ST_BufferClusteringRequiredDistance(
   geom GEOMETRY,
   required_distance float,
   gr_cnt integer)

CREATE OR REPLACE FUNCTION ST_BufferClusteringRequiredDistance(
   geom GEOMETRY,
   required_distance float,
   gr_cnt integer)
RETURNS GEOMETRY AS
$BODY$
      WITH
          tbla AS (SELECT (ST_Dump(geom)).geom geom),
          tblb AS (SELECT ((ST_Dump(ST_Union(ST_Buffer(geography(geom), required_distance)::geometry))).geom) geom FROM tbla),
          tblc AS (SELECT b.geom, count (*) cnt FROM tbla a JOIN tblb b ON ST_Intersects(ST_PointOnSurface(a.geom), b.geom) GROUP BY b.geom),
          tbld AS (SELECT geom FROM tblc WHERE cnt NOT IN (gr_cnt))
          SELECT ST_Collect(a.geom) geom FROM tbla a WHERE EXISTS (SELECT 1 FROM tbld b WHERE ST_Intersects(ST_PointOnSurface(a.geom), b.geom));
$BODY$
LANGUAGE SQL

SELECT ST_BufferClusteringRequiredDistance(geom, 501, 1) geom FROM (SELECT ST_Collect(geom) geom FROM <polygons_name_table) foo