7. Writing a SQL Stored Procedures

../_images/route.png

pgRouting functions provide low level interface. When developing for a higher level application, the requirements need to be represented in the SQL queries. As these SQL queries get more complex, it is desirable to store them in postgreSQL stored procedures or functions. Stored procedures are an effective way to wrap application logic, in this case, related to routing logic.

7.1. The application requirements

The stored procedure that is going to be developed has the following requirements:

  1. Vehicles are routed.
  2. Starting and ending vertices are by selection using osm_id.

    • In past chapters it was done using the id of the vertices.

  3. Name of the road on the path.

  4. The geometry segments along the route path with the correct orientation.

    • Geometry is to be returned.

    • Azimuth in degrees of the geometry is to be returned.

    • Geometry handling is needed to get the correct orientation.

7.1.1. Exercise 1 - Segments for Vehicle Routing

View of roads for vehicles

The vehicle can not circulate on non-pedestrian roads

  • Create a view of the allowed road network for circulation.

  • Routing costs will be based on minutes.

  • Verify the reduced number of road segments.

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-- DROP VIEW vehicle_net CASCADE;

CREATE VIEW vehicle_net AS
    SELECT gid,
        source,
        target,
        -- converting to minutes
        cost_s / 60 AS cost,
        reverse_cost_s / 60 AS reverse_cost,
        the_geom
    FROM ways JOIN configuration AS c
    USING (tag_id)
    WHERE  c.tag_value NOT IN ('steps','footway','path');

-- Verification
SELECT count(*) FROM ways;
SELECT count(*) FROM vehicle_net;

Solution to Chapter 7 Exercise 1

7.1.2. Exercise 2 - Limiting the Road Network within an Area

View of smaller set of roads for vehicles

The vehicle can only circulate inside this Bounding Box: (26.08, 44.42, 26.11, 44.44)

  • The vehicle can only circulate inside the bounding box: (26.08, 44.42, 26.11, 44.44)

  • Create a view of the allowed road network for circulation.

  • Use the vehicle_net VIEW.

  • Verify the reduced number of road segments.

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-- DROP VIEW little_net;

CREATE VIEW little_net AS
    SELECT *
    FROM vehicle_net
    WHERE vehicle_net.the_geom && ST_MakeEnvelope(26.08, 44.42, 26.11, 44.44);

-- Verification
SELECT count(*) FROM little_net;

Solution to Chapter 7 Exercise 2

7.1.3. Exercise 3 - Route using “osm_id”

From the Venue to the hotel using the osm_id.

From the Venue to the hotel using the osm_id.

  • The vehicle is going from the Venue at 6498351588.

  • The vehicle is going to the hotel at 255093299.

  • Start and end vertex are given with their osm_id.

  • The result should contain:

    • seq for ordering and unique row identifier

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SELECT *
FROM pgr_dijkstra(
    'SELECT gid AS id, * FROM vehicle_net',
    -- source
    (SELECT id FROM ways_vertices_pgr WHERE osm_id = 6498351588),
    -- target
    (SELECT id FROM ways_vertices_pgr WHERE osm_id = 255093299));

Solution to Chapter 7 Exercise 3

7.1.4. Exercise 4 - Get additional information

Route showing names

From the venue at National Theater Bucharest to the Hotel Capitol, additionally get the name of the roads.

  • The vehicle is going from the venue at National Theater Bucharest at 6498351588.

  • The vehicle is going to the Hotel Capitol at 255093299.

  • The result should contain:

    • seq for ordering and unique row identifier

    • the name of the road segments

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SELECT dijkstra.*, ways.name
FROM pgr_dijkstra(
    'SELECT gid AS id, * FROM vehicle_net',
    (SELECT id FROM ways_vertices_pgr WHERE osm_id = 6498351588),
    (SELECT id FROM ways_vertices_pgr WHERE osm_id = 255093299)
    ) AS dijkstra
LEFT JOIN ways
ON (edge = gid) ORDER BY seq;

Solution to Chapter 7 Exercise 4

7.2. Geometry handling

7.2.1. Exercise 5 - Route geometry (human readable)

From the venue at National Theater Bucharest to the Hotel Capitol, additionally get the geometry in human readable form.

From the Venue to the Brewry

  • The vehicle is going from the venue at National Theater Bucharest at 6498351588

  • The vehicle is going to the Hotel Capitol at 255093299.

  • The result should contain:

    • seq for ordering and unique row identifier

    • the name of the road segments

    • the geometry of the path in human readable form.

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SELECT dijkstra.*, ways.name, ST_AsText(ways.the_geom)
FROM pgr_dijkstra(
    'SELECT gid AS id, * FROM vehicle_net',
    (SELECT id FROM ways_vertices_pgr WHERE osm_id = 6498351588),
    (SELECT id FROM ways_vertices_pgr WHERE osm_id = 255093299)
    ) AS dijkstra
LEFT JOIN ways
ON (edge = gid) ORDER BY seq;

Solution to Chapter 7 Exercise 5

Note

The last row of the result, does not contain a geometry value since the shortest path function returns -1 for the last edge to indicate the end of the route.

7.2.2. Exercise 6 - Route geometry (binary format)

From |place_3| to the |place_1| showing arrows.

From the venue at National Theater Bucharest to the Hotel Capitol by car, get the binary format geometry that can be used by a front end app.

Note

Not using ST_AsText gives the binary format.

Tip

WITH provides a way to write auxiliary statements in larger queries. It can be thought of as defining temporary tables that exist just for one query.

  • The vehicle is going from the venue at National Theater Bucharest at 6498351588.

  • The vehicle is going to the Hotel Capitol at 255093299.

  • The result should contain:

    • seq for ordering and unique row identifier.

    • the name of the road segments.

    • the geometry of the path in human readable form.

    • the geometry of the path in default binary format.

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WITH
dijkstra AS (
    SELECT * FROM pgr_dijkstra(
        'SELECT gid AS id, * FROM vehicle_net',
        (SELECT id FROM ways_vertices_pgr WHERE osm_id = 6498351588),
        (SELECT id FROM ways_vertices_pgr WHERE osm_id = 255093299))
)
SELECT dijkstra.*, ways.name, ways.the_geom AS route_geom
FROM dijkstra LEFT JOIN ways ON (edge = gid)
ORDER BY seq;

Solution to Chapter 7 Exercise 6

7.2.3. Exercise 7 - Using the geometry

From |place_3| to the |place_1| show azimuth

From the venue at National Theater Bucharest to the Hotel Capitol, calculate the azimuth in degrees.

  • The vehicle is going from the venue at National Theater Bucharest at 6498351588.

  • The vehicle is going to the Hotel Capitol at 255093299.

  • Get the seq, name, cost, azimuth in degrees and the geometry.

  • The geometry of the route path in human readable form & binary form.

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WITH
dijkstra AS (
    SELECT * FROM pgr_dijkstra(
        'SELECT gid AS id, * FROM vehicle_net',
        (SELECT id FROM ways_vertices_pgr WHERE osm_id = 6498351588),
        (SELECT id FROM ways_vertices_pgr WHERE osm_id = 255093299))
),
get_geom AS (
    SELECT dijkstra.*, ways.name, ways.the_geom AS route_geom
    FROM dijkstra JOIN ways ON (edge = gid)
    ORDER BY seq)
SELECT seq, name, cost,
    -- calculating the azimuth
    degrees(ST_azimuth(ST_StartPoint(route_geom), ST_EndPoint(route_geom))) AS azimuth,
    ST_AsText(route_geom),
    route_geom
FROM get_geom
ORDER BY seq;

Solution to Chapter 7 Exercise 7

7.2.4. Exercise 8 - Geometry directionality

From |place_3| to the |place_1|

From the venue at National Theater Bucharest, going to the Hotel Capitol by car, get the geometry with correct arrow directionality.

When we generate a route the segments are returned as the geometry in the database. It means that the segments can be reversed relative to the direction of the route path. Our goal is to have all segments oriented correctly along the route path.

  • The vehicle is going from the venue at National Theater Bucharest at 6498351588.

  • The vehicle is going to the Hotel Capitol at 255093299.

  • The first point of the segment must “match” with the last point of the previous segment.

  • Get the seq, name, cost, azimuth and the geomtery.

  • The geometry of the route path in human readable form & binary form.

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WITH
dijkstra AS (
    SELECT * FROM pgr_dijkstra(
        'SELECT gid AS id, * FROM vehicle_net',
        (SELECT id FROM ways_vertices_pgr WHERE osm_id = 6498351588),
        (SELECT id FROM ways_vertices_pgr WHERE osm_id = 255093299))
),
get_geom AS (
    SELECT dijkstra.*, ways.name,
        -- adjusting directionality
        CASE
            WHEN dijkstra.node = ways.source THEN the_geom
            ELSE ST_Reverse(the_geom)
        END AS route_geom
    FROM dijkstra JOIN ways ON (edge = gid)
    ORDER BY seq)
SELECT seq, name, cost,
    degrees(ST_azimuth(ST_StartPoint(route_geom), ST_EndPoint(route_geom))) AS azimuth,
    ST_AsText(route_geom),
    route_geom
FROM get_geom
ORDER BY seq;

Solution to Chapter 7 Exercise 8

Note

Comparing row 10 & 11 from Solution to Chapter 7 Exercise 5

-- from Exercise 5
LINESTRING(26.1007594 44.4390039,26.1006676 44.4391489)
LINESTRING(26.1004837 44.4391168,26.1006676 44.4391489)

-- from Excercise 8
LINESTRING(26.1007594 44.4390039,26.1006676 44.4391489)
LINESTRING(26.1006676 44.4391489,26.1004837 44.4391168)

  • In Exercise 5 the first point of row 11 does not match the last point of row 10

  • In Exercise 8 the first point of row 11 matches the last point of row 10

7.3. Creating a Function

The following function simplifies (and sets default values) when it calls the shortest path Dijkstra function.

Tip

pgRouting uses heavely function overloading:

  • Avoid the name of a function installed with pgRouting

  • Avoid the name of a function starting with pgr_ & ST_

7.3.1. Exercise 9 - Function for an application

Putting all together in a SQL function

  • Should work for any given area.

  • Data tables:

    • The edges are found in ways.

    • The vertices are found in ways_vertices_pgr.

  • Allow a view as a parameter

    • A table can be used if the columns have the correct names.

  • Start and end vertex are given with their osm_id.

  • The result should contain:

    • seq, name, cost, azimuth and the geometry.

    • The geometry of the route path in human readable form & binary form.

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--DROP FUNCTION wrk_dijkstra(regclass, bigint, bigint);

CREATE OR REPLACE FUNCTION wrk_dijkstra(
        IN edges_subset regclass,
        IN source BIGINT,
        IN target BIGINT,
        OUT seq INTEGER,
        OUT gid BIGINT,
        OUT name TEXT,
        OUT cost FLOAT,
        OUT azimuth FLOAT,
        OUT route_readable TEXT,
        OUT route_geom geometry
    )
    RETURNS SETOF record AS
$BODY$
    WITH
    dijkstra AS (
        SELECT * FROM pgr_dijkstra(
            -- using parameters instead of specific values
            'SELECT gid AS id, * FROM ' || $1,
            (SELECT id FROM ways_vertices_pgr WHERE osm_id = $2),
            (SELECT id FROM ways_vertices_pgr WHERE osm_id = $3))
    ),
    get_geom AS (
        SELECT dijkstra.*, ways.name,
            CASE
                WHEN dijkstra.node = ways.source THEN the_geom
                ELSE ST_Reverse(the_geom)
            END AS route_geom
        FROM dijkstra JOIN ways ON (edge = gid)
        ORDER BY seq)
    SELECT
        seq,
        edge,  -- will get the name "gid"
        name,
        cost,
        degrees(ST_azimuth(ST_StartPoint(route_geom), ST_EndPoint(route_geom))) AS azimuth,
        ST_AsText(route_geom),
        route_geom
    FROM get_geom
    ORDER BY seq;
$BODY$
LANGUAGE 'sql';

Solution to Chapter 7 Exercise 9

7.3.2. Exercise 10 - Using the function

  • The osm_id must exist on the ways_vertices_pgr table.

  • If an osm_id falls outside the view, No path will be returned.

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SELECT *
FROM wrk_dijkstra('vehicle_net',  6498351588, 255093299);

Solution to Chapter 7 Exercise 10

Note

Try the function with little_net and a combination of the interesting places:

  • 255093299 Hotel Capitol

  • 6159253045 Little Bucharest Hostal

  • 6498351588 venue at National Theater Bucharest

  • 123392877 workshops at Faculty of Geography of the University of Bucharest

  • 1886700005 Parliament House

7.3.3. Exercise 11 - Saving the function

Save the function code above into a file ~/Desktop/workshop/wrk_dijkstra.sql.

Saving functions in a file can be used to install the function in another database. Install the function into the database with:

psql -U user -d city_routing -f ~/Desktop/workshop/wrk_dijkstra.sql