PostGis 最近邻查询

Question

我想检索另一组点的给定范围内的所有点。比方说，找到任何地铁站 500 米范围内的所有商店。

我写了这个查询，很慢，想优化一下：

SELECT DISCTINCT ON(locations.id) locations.id FROM locations, pois
WHERE pois.poi_kind = 'subway'
AND ST_DWithin(locations.coordinates, pois.coordinates, 500, false);

我运行使用最新版本的 Postgres 和 PostGis（Postgres 9.5、PostGis 2.2.1）

这是 table 元数据：

                                         Table "public.locations"
       Column       |            Type             |                       Modifiers
--------------------+-----------------------------+--------------------------------------------------------
 id                 | integer                     | not null default nextval('locations_id_seq'::regclass)
 coordinates        | geometry                    |
Indexes:
    "locations_coordinates_index" gist (coordinates)


                                      Table "public.pois"
   Column    |            Type             |                     Modifiers
-------------+-----------------------------+---------------------------------------------------
 id          | integer                     | not null default nextval('pois_id_seq'::regclass)
 coordinates | geometry                    |
 poi_kind_id | integer                     |
Indexes:
    "pois_pkey" PRIMARY KEY, btree (id)
    "pois_coordinates_index" gist (coordinates)
    "pois_poi_kind_id_index" btree (poi_kind_id)
Foreign-key constraints:
    "pois_poi_kind_id_fkey" FOREIGN KEY (poi_kind_id) REFERENCES poi_kinds(id)

这是 EXPLAIN (ANALYZE, BUFFERS) 的结果：

Unique  (cost=2407390.71..2407390.72 rows=2 width=4) (actual time=3338.080..3338.252 rows=918 loops=1)
Buffers: shared hit=559
->  Sort  (cost=2407390.71..2407390.72 rows=2 width=4) (actual time=3338.079..3338.145 rows=963 loops=1)
      Sort Key: locations.id
      Sort Method: quicksort  Memory: 70kB
      Buffers: shared hit=559
      ->  Nested Loop  (cost=0.00..2407390.71 rows=2 width=4) (actual time=2.466..3337.835 rows=963 loops=1)
            Join Filter: (((pois.coordinates)::geography && _st_expand((locations.coordinates)::geography, 500::double precision)) AND ((locations.coordinates)::geography && _st_expand((pois.coordinates)::geography, 500::double precision)) AND _st_dwithin((pois.coordinates)::geography, (locations.coordinates)::geography, 500::double precision, false))
            Rows Removed by Join Filter: 4531356
            Buffers: shared hit=559
            ->  Seq Scan on locations  (cost=0.00..791.68 rows=24168 width=36) (actual time=0.005..3.100 rows=24237 loops=1)
                  Buffers: shared hit=550
            ->  Materialize  (cost=0.00..10.47 rows=187 width=32) (actual time=0.000..0.009 rows=187 loops=24237)
                  Buffers: shared hit=6
                  ->  Seq Scan on pois  (cost=0.00..9.54 rows=187 width=32) (actual time=0.015..0.053 rows=187 loops=1)
                        Filter: (poi_kind_id = 3)
                        Rows Removed by Filter: 96
                        Buffers: shared hit=6
Planning time: 0.184 ms
Execution time: 3338.304 ms
(20 rows)

Answer 1

我觉得你应该换个解决方案，postgis还是运行结构化数据库中的查询，功能强大，但在特殊需求下速度不快，可能你需要elasticsearch。

elasticsearch 擅长地理位置搜索，但不擅长地理数据处理，我想你两者都需要。

https://www.elastic.co/blog/geo-location-and-search

Answer 2

由于第四个参数，我认为您使用的是 st_dwithin 的地理版本。

尝试将您的查询更改为以下查询：

SELECT DISCTINCT ON(locations.id) locations.id FROM locations, pois
WHERE pois.poi_kind = 'subway'
AND ST_DWithin(locations.coordinates, pois.coordinates, 500);

如果还是没有解决，请post重新解释分析。

Answer 3

我最终得出结论，我无法在现实的时间内（< 1 秒）实时计算数千个兴趣点和数千个位置之间的距离。

所以我预先计算了所有内容：每次位置或 POI 是 created/updated，我存储每个位置和每种 POI 之间的最小距离，以便能够回答问题 "which locations are closer than X meters from this kind of POI".

这是我为此编写的模块（它在 Elixir 中，但主要部分是原始的 SQL）

defmodule My.POILocationDistanceService do

  alias Ecto.Adapters.SQL
  alias My.Repo

  def delete_distance_for_location(location_id) do
    run_query!("DELETE FROM poi_location_distance WHERE location_id = ::integer", [location_id])
  end

  def delete_distance_for_poi_kind(poi_kind_id) do
    run_query!("DELETE FROM poi_location_distance WHERE poi_kind_id = ::integer", [poi_kind_id])
  end

  def insert_distance_for_location(location_id) do
    sql = """
    INSERT INTO poi_location_distance(poi_kind_id, location_id, poi_id, distance)
    SELECT
      DISTINCT ON (p.poi_kind_id)
      p.poi_kind_id as poi_kind_id,
      l.id as location_id,
      p.id as poi_id,
      MIN(ST_Distance_Sphere(l.coordinates, p.coordinates)) as distance
    FROM locations l, pois p
    WHERE
      l.id = 
      AND ST_DWithin(l.coordinates, p.coordinates, , FALSE)
    GROUP BY p.poi_kind_id, p.id, l.id
    ORDER BY p.poi_kind_id, distance;
    """

    run_query!(sql, [location_id, max_distance])
  end

  def insert_distance_for_poi_kind(poi_kind_id, offset \ 0, limit \ 10_000_000) do
    sql = """
    INSERT INTO poi_location_distance(poi_kind_id, location_id, poi_id, distance)
    SELECT
      DISTINCT ON(l.id, p.poi_kind_id)
      p.poi_kind_id as poi_kind_id,
      l.id as location_id,
      p.id as poi_id,
      MIN(ST_Distance_Sphere(l.coordinates, p.coordinates)) as distance
    FROM pois p, (SELECT * FROM locations OFFSET  LIMIT ) as l
    WHERE
      p.poi_kind_id = 
      AND ST_DWithin(l.coordinates, p.coordinates, , FALSE)
    GROUP BY l.id, p.poi_kind_id, p.id;
    """

    run_query!(sql, [offset, limit, poi_kind_id, max_distance])
  end

  defp run_query!(query, params) do
    SQL.query!(Repo, query, params)
  end

  def max_distance, do: 5000

end

PostGis 最近邻查询

PostGis nearest neighbours query

sql

postgresql

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query-performance

nearest-neighbor