PLPGSQL函数来计算方位 [英] PLPGSQL Function to Calculate Bearing

问题描述

基本上，我无法理解plpgsql的语法，并且希望在以下工作中有所帮助. 我有一个包含1000的wgs84点的表.以下SQL将在此表的边界框中检索一组点:

Basically I can't get my head around the syntax of plpgsql and would appreciate some help with the following efforts. I have a table containing 1000's of wgs84 points. The following SQL will retrieve a set of points within a bounding box on this table:

SELECT id, ST_X(wgs_geom), ST_Y(wgs_geom), ST_Z(wgs_geom) 
  FROM points_table 
  INNER JOIN 
   (SELECT ST_Transform(ST_GeomFromText('POLYGON((-1.73576102027 1.5059743629,
     -1.73591122397 51.5061067655,-1.73548743495 51.5062838333,-1.73533186682 
      1.5061514313,-1.73576102027 51.5059743629))', 4326, 27700)
   ) AS bgeom
  ) AS t2 
ON ST_Within(local_geom, t2.bgeom)

我需要做的是在结果中添加一个方位/方位角列，该列描述返回数据集中每个点的方位. 因此，我尝试实现的方法是构建一个plpgsql函数，该函数可以按上述方式选择数据并计算循环中每组点之间的方位. 但是，我在理解plpgsql函数中的基本数据访问和处理方面的努力却以失败告终.

What I need to do is add a bearing/azimuth column to the results that describes the bearing at each point in the returned data set. So the approach I'm trying to implement is to build a plpgsql function that can select the data as per above and calculate the bearing between each set of points in a loop. However my efforts at understanding basic data access and handling within a plpgsql function are failing miserably.

我要创建的函数的当前版本的示例如下:

An example of the current version of the function I'm trying to create is as follows:

CREATE TYPE bearing_type AS (x numeric, y numeric, z numeric, bearing numeric);

--DROP FUNCTION IF EXISTS get_bearings_from_points();

CREATE OR REPLACE FUNCTION get_bearings_from_points()
   RETURNS SETOF bearing_type AS

$BODY$

DECLARE
    rowdata points_table%rowtype;
    returndata bearing_type;

BEGIN
    FOR rowdata IN
        SELECT nav_id, wgs_geom 
          FROM points_table INNER JOIN 
             (SELECT ST_Transform(ST_GeomFromText('POLYGON((-1.73576102027
                3.5059743629,-1.73591122397 53.5061067655,-1.73548743495 
                53.5062838333,-1.73533186682 53.5061514313,-1.73576102027 
                53.5059743629))', 4326), 27700) 
             AS bgeom) 
          AS t2 ON ST_Within(local_geom, t2.bgeom)
    LOOP
        returndata.x := ST_X(rowdata.wgs_geom);
        returndata.y := ST_Y(rowdata.wgs_geom);
        returndata.z := ST_Z(rowdata.wgs_geom);
        returndata.bearing := ST_Azimuth(<current_point> , <next_point>)
    RETURN NEXT returndata;
    END LOOP;
    RETURN;
END

$BODY$
LANGUAGE plpgsql;

我只想按以下方式调用此函数:

I would like to just call this function as follows:

SELECT get_bearings_from_points();

并获得所需的结果. 基本上，问题在于了解如何正确访问行数据，以便我可以读取当前点和下一点.

and get the desired result. Basically the problems are understanding how to access the rowdata properly such that I can read the current and next points.

在上面的示例中，我遇到了各种问题，例如如何调用ST_X等SQL函数，并尝试了EXECUTE select语句，这些错误带有关于几何数据类型的错误.

In the above example I've had various problems from how to call the ST_X etc SQL functions and have tried EXECUTE select statements with errors re geometry data types.

任何见解/帮助将不胜感激.

Any insights/help would be much appreciated.

推荐答案

在PL/pgSQL中，一次完成尽可能多的基本SQL查询操作是最有效的.您可以大大简化.

In PL/pgSQL it's most effective to do as much as is elegantly possible in basic SQL queries at once. You can largely simplify.

我没有从您的问题中得到排序顺序的定义，而是留下???替您填写:

I didn't get a definition of the sort order out of your question and left ??? to fill in for you:

CREATE OR REPLACE FUNCTION get_bearings_from_points(_bgeom geometry)
  RETURNS TABLE (x numeric, y numeric, z numeric, bearing numeric) AS
$func$
BEGIN
   FOR x, y, z, bearing IN
      SELECT ST_X(t.wgs_geom), ST_Y(t.wgs_geom), ST_Z(t.wgs_geom)
           , ST_Azimuth(t.wgs_geom, lead(t.wgs_geom) OVER (ORDER BY ???))
      FROM   points_table t
      WHERE  ST_Within(t.local_geom, _bgeom)
      ORDER  BY ???
   LOOP
      RETURN NEXT;
   END LOOP;
END
$func$ LANGUAGE plpgsql;

窗口函数 根据排序顺序从下一行引用一列.

The window function lead() references a column from the next row according to sort order.

这可以简化为单个SQL查询-可能包装为SQL函数:

This can be simplified further to a single SQL query - possibly wrapped into an SQL function:

CREATE OR REPLACE FUNCTION get_bearings_from_points(_bgeom geometry)
  RETURNS TABLE (x numeric, y numeric, z numeric, bearing numeric) AS
$func$
   SELECT ST_X(t.wgs_geom), ST_Y(t.wgs_geom), ST_Z(t.wgs_geom)
        , ST_Azimuth(t.wgs_geom, lead(t.wgs_geom) OVER (ORDER BY ???))
   FROM   points_table t
   WHERE  ST_Within(t.local_geom, $1)  -- use numbers in pg 9.1 or older
   ORDER  BY ???
$func$ LANGUAGE sql;

参数名称可以在pg 9.2或更高版本中引用. 第9.2页的发行说明:

Parameter names can be referenced in pg 9.2 or later. Per release notes of pg 9.2:

允许SQL语言函数按名称引用参数(马修 德雷珀(Draper)

Allow SQL-language functions to reference parameters by name (Matthew Draper)

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