openstreetmap - PGROuting 2 : Getting nearest node to a given point (with lat - long given)

I have a point whose latitude and longitude (only) are known, and a table of nodes with their geom. I want to find the closest node in the table to the given point.

I think I can achieve this with pgr_findNearestNodeDwithin function, but i cannot find the exact syntax of the function and the parameters it takes.

Any pointers in this direction will be appreciated.

Answer

To find the nearest point you only need a little bit PostGIS. With PostGIS 2.0 or higher you can use nearest neighbor KNN gist functionality.

I assume you have a table with all nodes ("vertices"):

SELECT * FROM vertices 
    ORDER BY the_geom <-> ST_GeometryFromText('POINT(lon lat)',4326) 
    LIMIT 1;