# Driving Distance calculation

[日本語]

The driving_distance function has the following signature:

CREATE OR REPLACE FUNCTION driving_distance(sql text, source_id integer, distance float8, directed boolean, has_reverse_cost boolean) RETURNS SETOF path_result

arguments are:

**sql**: a SQL query, which should return a set of rows with the following columns:

- id: an int4 identifier of the edge
- source: an int4 identifier of the source vertex
- target: an int4 identifier of the target vertex
- cost: an float8 value, of the edge traversal cost. (a negative cost will prevent the edge from being inserted in the graph).

**source_id**: int4 id of the start point distance: float8 value of distance in degrees

**distance**: float8 value in edge cost units (not in projection units - they might be different).

**directed**: true if the graph is directed

**has_reverse_cost**: if true, the reverse_cost column of the SQL generated set of rows will be used for the cost of the traversal of the edge in the opposite direction.

The function returns a set of rows. There is one row for each crossed edge, and an additional one containing the terminal vertex. The columns of each row are:

- vertex_id: the identifier of source vertex of each edge. There is one more row after the last edge, which contains the vertex identifier of the target path.
- edge_id: the identifier of the edge crossed
- cost: The cost associated to the current edge. It is 0 for the row after the last edge. Thus, the path total cost can be computated using a sum of all rows in the cost column.

## Examples:

SELECT * FROM driving_distance('SELECT gid AS id,source,target, length::double precision AS cost FROM dourol',10549,0.01,false,false);

vertex_id | edge_id | cost -----------+---------+--------------- 6190 | 120220 | 0.00967666852 6205 | 118671 | 0.00961557335 6225 | 119384 | 0.00965668162 6320 | 119378 | 0.00959826176 ... ... ... 15144 | 122612 | 0.00973386526 15285 | 120471 | 0.00912965866 15349 | 122085 | 0.00944814966 15417 | 120471 | 0.00942316736 15483 | 121629 | 0.00972957546 (293 rows)