PLPGSQL Function to Calculate Bearing - sql

Question

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)
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.
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;
I would like to just call this function as follows:
SELECT get_bearings_from_points();
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.
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.
Any insights/help would be much appreciated.

Answer 1

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.
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;
The window function lead() references a column from the next row according to sort order.
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;
Parameter names can be referenced in pg 9.2 or later. Per release notes of pg 9.2:
Allow SQL-language functions to reference parameters by name (Matthew
Draper)