new doc

Author: digoal

202004/20200410_02.md

@@ -14,6 +14,8 @@ PostgreSQL , plan , pg_stat_statements
 ## 背景    
 https://rjuju.github.io/postgresql/2020/04/04/new-in-pg13-monitoring-query-planner.html  
 
+https://www.depesz.com/2020/04/02/waiting-for-postgresql-13-allow-pg_stat_statements-to-track-planning-statistics/  
+  
 PG 13 pg_stat_statements支持跟踪plan统计信息, 有什么用?  
 
 1、OLTP场景, 并发高, sql简单同时基本上同一sql可以使用一个单一执行计划, 除非数据发生倾斜(PG的绑定变量可以通过custom plan算法解决), 所以让数据库尽量少的开销浪费在执行计划上.  

202004/20200412_01.md

@@ -0,0 +1,101 @@
+## PostgreSQL Polygon Averaging in PostGIS - 多边形叠加统计  
+      
+### 作者      
+digoal      
+      
+### 日期      
+2020-04-12      
+      
+### 标签      
+PostgreSQL , postgis , 多边形 , 叠加 , 统计    
+      
+----      
+      
+## 背景      
+https://info.crunchydata.com/blog/polygon-averaging-in-postgis  
+    
+Let's say we received polygons from 10 users. If we used ST_Intersection on those polygons, the remaining polygon would only represent the points included in all 10 polygons. If we used ST_Union, the output would represent the points included in at least 1 polygon.  
+  
+Can anyone recommend a way to output a polygon that represents the points that are in n polygons, where n is greater than 1 and less than the total number of polygons (10 in this case)?  
+  
+这个问题是要知道在多个多边形叠加后, 在多边形内取一个点, 这个点到底被多少个多边形覆盖到了, 切出一个个的面, 每个面有多少个多边形叠加.   
+  
+![pic](20200412_01_pic_001.png)  
+  
+```  
+WITH   
+edges AS   
+  ( SELECT  
+    (ST_Dump(  
+     ST_UnaryUnion(  
+     ST_Collect(  
+     ST_ExteriorRing(p.geom))))).geom  
+  FROM polygons p )  
+```  
+  
+This requires a few functions:  
+  
+- [ST_ExteriorRing](https://postgis.net/docs/ST_ExteriorRing.html) to get the polygon boundary,  
+- [ST_UnaryUnion](https://postgis.net/docs/ST_UnaryUnion.html) to mash all the rings together and break them at crossing points, and S  
+- [ST_Dump](https://postgis.net/docs/ST_Dump.html) to convert the output of the union (a multi-linestring) back into one row per geometry.  
+  
+![pic](20200412_01_pic_002.png)  
+  
+```  
+parts AS (  
+  SELECT (ST_Dump(ST_Polygonize(geom))).geom FROM edges  
+)  
+  
+  
+parts_count AS (  
+  SELECT parts.geom, count(*)  
+  FROM parts  
+  JOIN polygons p  
+  ON ST_Intersects(p.geom, ST_PointOnSurface(parts.geom))  
+  GROUP BY parts.geom  
+)  
+```  
+  
+![pic](20200412_01_pic_003.png)  
+  
+```  
+SELECT ST_Union(geom) AS geom, 4 as count  
+FROM parts_count  
+WHERE count > 4  
+```  
+  
+![pic](20200412_01_pic_004.png)  
+  
+```  
+CREATE TABLE average AS  
+WITH  
+edges AS (  
+    SELECT (ST_Dump(ST_UnaryUnion(ST_Collect(ST_ExteriorRing(p.geom))))).geom  
+    FROM polygons2 p  
+),  
+parts AS (  
+    SELECT (ST_Dump(ST_Polygonize(geom))).geom FROM edges  
+),  
+parts_count AS (  
+    SELECT parts.geom, count(*)  
+    FROM parts  
+    JOIN polygons2 p  
+    ON ST_Intersects(p.geom, ST_PointOnSurface(parts.geom))  
+    GROUP BY parts.geom  
+)  
+SELECT ST_Union(geom) AS geom, 4 as count  
+FROM parts_count  
+WHERE count > 4  
+```  
+  
+![pic](20200412_01_pic_005.png)  
+  
+  
+#### [免费领取阿里云RDS PostgreSQL实例、ECS虚拟机](https://www.aliyun.com/database/postgresqlactivity "57258f76c37864c6e6d23383d05714ea")
+  
+  
+#### [digoal's PostgreSQL文章入口](https://github.com/digoal/blog/blob/master/README.md "22709685feb7cab07d30f30387f0a9ae")
+  
+  
+![digoal's weixin](../pic/digoal_weixin.jpg "f7ad92eeba24523fd47a6e1a0e691b59")
+  

202004/20200412_01_pic_001.png

@@ -2,6 +2,7 @@
 
 ### 文章列表  
 ----  
+##### 20200412_01.md   [《PostgreSQL Polygon Averaging in PostGIS - 多边形叠加统计》](20200412_01.md)  
 ##### 20200410_02.md   [《PostgreSQL 13 pg_stat_statements支持跟踪plan统计信息》](20200410_02.md)  
 ##### 20200410_01.md   [《PostgreSQL 13 支持wal insert统计框架 - wal activity monitor》](20200410_01.md)  
 ##### 20200408_01.md   [《为什么pgpool sr_check_password为空也能认证成功》](20200408_01.md)  

202004/20200412_01_pic_002.png

@@ -54,6 +54,7 @@ digoal's|PostgreSQL|文章|归类
 
 ### 所有文档如下  
 ----  
+##### 202004/20200412_01.md   [《PostgreSQL Polygon Averaging in PostGIS - 多边形叠加统计》](202004/20200412_01.md)  
 ##### 202004/20200410_02.md   [《PostgreSQL 13 pg_stat_statements支持跟踪plan统计信息》](202004/20200410_02.md)  
 ##### 202004/20200410_01.md   [《PostgreSQL 13 支持wal insert统计框架 - wal activity monitor》](202004/20200410_01.md)  
 ##### 202004/20200408_01.md   [《为什么pgpool sr_check_password为空也能认证成功》](202004/20200408_01.md)