问题

书接上文点抽稀，在加载低层级瓦片时，发现瓦片体积增大，渲染压力也比较大。

思路

在上文点抽稀的基础上稍作改造，提取多边形中心点如法炮制，抽稀的结果再和原多边形匹配，返回抽稀后的多边形即可

SQL

select 
  b.geom, 
  a.count, 
  b.name 
from 
  (
    select 
      --用于分组
      width_bucket(
        st_x(c.geom), 
        st_xmin(
          ST_TileEnvelope(Z, X, Y)
        ), 
        st_xmax(
          ST_TileEnvelope(Z, X, Y)
        ), 
        50
      ) as grid_x, 
      width_bucket(
        st_y(c.geom), 
        st_ymin(
          ST_TileEnvelope(Z, X, Y)
        ), 
        st_ymax(
          ST_TileEnvelope(Z, X, Y)
        ), 
        50
      ) as grid_y, 
      --集合&amp;选点&amp;统计
      ST_GeometryN(
        st_collect(c.geom), 
        1
      ) as geom, 
      count(c.geom) 
    from 
      (
        select 
          st_centroid(geom) as geom 
        from 
          buildings
      ) as c --中心点拿出来抽稀
    where 
      st_intersects(
        c.geom, 
        ST_TileEnvelope(Z, X, Y)
      ) 
    group by 
      grid_x, 
      grid_y
  ) a, 
  buildings b 
where 
  a.geom && st_centroid(b.geom); --重新匹配原有几何