After pgr_createTopology, how should I use pgr_dijkstra?

Question

Using the OpenStreetMap data and osm2pgsql, I created a topology by using the pgr_createTopology() function. These tables are what I have:

                          List of relations
 Schema |                 Name                 |   Type   |  Owner   
--------+--------------------------------------+----------+----------
 public | geography_columns                    | view     | postgres
 public | geometry_columns                     | view     | postgres
 public | planet_osm_line                      | table    | postgres
 public | planet_osm_nodes                     | table    | postgres
 public | planet_osm_point                     | table    | postgres
 public | planet_osm_polygon                   | table    | postgres
 public | planet_osm_rels                      | table    | postgres
 public | planet_osm_roads                     | table    | postgres
 public | planet_osm_roads_vertices_pgr        | table    | postgres
 public | planet_osm_roads_vertices_pgr_id_seq | sequence | postgres
 public | planet_osm_ways                      | table    | postgres
 public | spatial_ref_sys                      | table    | postgres
(12 rows)

Now I want to run pgr_dijkstra() to find the shortest path.

Which table and columns should I use? Which column represents start_vid and end_vid?

Answer 1

Like explain in the documentation the pgr_dijkstra function need the following parameters :

1. Edges SQL : A string that represents a SQL query for extract the edges from your edge_table.
2. start_vid : An integer or an array of integers corresponding to the id (in the vertices tables ) of the start point(s).
3. end_vid: An integer or an array of integers corresponding to the id (in the vertices tables ) of the end point(s).
4. (optional) directed : A boolean which indicates if the graph is directed (in this case you need a reverse_cost column in your edge_table.

If your edge_table has the default names the query will look like this :

SELECT *
FROM pgr_dijkstra(
    'SELECT id, source, target, cost, reverse_cost FROM edge_table',
    2, 
    3,
    FALSE
);

NB: For the query to work, a positive cost must be assigned to each section (the function pgr_createTopology does not), for example with the following query :

UPDATE edge_table SET cost = St_length(geom);