Why is pgr_dijkstra not giving shortest path

Question

I am using a road network and the following postgres/postgis operations to perform routing using pg_routing and the pgr_dijkstra method. I am successfully generating routes but they are not the shortest. Here are my methods..

Uploaded road data to postgis.
Added columns source and target.

alter table roads_4326 add column source integer;
alter table roads_4326 add column target integer;

Created topology.

select pgr_createTopology('roads_4326', .0001, 'geom', 'id');

Which successfully produces the '..vertices_pgr' layer.

Then I run the following to route from source id=1 to target id=99.

SELECT seq, edge, geom
FROM pgr_dijkstra(
  'SELECT id,
      source, 
      target,
      st_length(geom) AS cost
    FROM roads_4326',
  1, 99,
  directed => false
) AS a
JOIN roads_4326 AS b ON (a.edge = b.id) ORDER BY seq;

A route is generated but I am unsure why much shorter routes are not returned. Image 1 is returned.

Why are the much shorter connections in Image 2 ignored?

Answer 1

Even though you have instructed to use an undirected graph, you must still use set a cost and a reverse_cost (which can be the same).

SELECT gid as id, source, target, 
        length:: double precision AS cost, 
        length:: double precision AS reverse_cost
 FROM bristol_roads

See the examples from the doc:

SELECT * FROM pgr_dijkstra(
    'SELECT id, source, target, cost, reverse_cost FROM edge_table',
    2, 3,
    FALSE
);
 seq | path_seq | node | edge | cost | agg_cost
-----+----------+------+------+------+----------
   1 |        1 |    2 |    2 |    1 |        0
   2 |        2 |    3 |   -1 |    0 |        1
(2 rows)

SELECT * FROM pgr_dijkstra(
    'SELECT id, source, target, cost FROM edge_table',
    2, 3,
    FALSE
);
 seq | path_seq | node | edge | cost | agg_cost
-----+----------+------+------+------+----------
   1 |        1 |    2 |    4 |    1 |        0
   2 |        2 |    5 |    8 |    1 |        1
   3 |        3 |    6 |    5 |    1 |        2
   4 |        4 |    3 |   -1 |    0 |        3
(4 rows)