Re: Best PostGIS function for finding the nearest line segment to a given point

2011/10/8 René Fournier <renefournier(at)gmail(dot)com>

> Wow, have to say, I love Postgresql and PostGIS. Just awesome.
>
> So I have a table with ~400,000 rows, each representing a road or street
> (multi line segment). I want to select the row whose line segment is closest
> the a given point. The following query...
>
> gc3=# SELECT r_stname_c, r_placenam,
> ST_Distance(ST_GeomFromText('POINT(-114.053205 51.069644)',4269),the_geom)
> AS distance FROM nrn_ab_8_0_roadseg ORDER BY distance ASC LIMIT 1;
>
>
> ...works and produces...
>
> r_stname_c | r_placenam | distance
> ----------------------+------------+----------------------
> 19 Avenue North-east | Calgary | 5.74515867479735e-05
>
> …but seems a little slow (yes, there is a GIST index on the_geom). Explain
> shows:
>
> gc3=# explain SELECT r_stname_c, r_placenam,
> ST_Distance(ST_GeomFromText('POINT(-114.053205 51.069644)',4269),the_geom)
> AS distance FROM nrn_ab_8_0_roadseg ORDER BY distance asc limit 1;
> QUERY PLAN
>
>
> -----------------------------------------------------------------------------------------------------------
> Limit (cost=128520.06..128520.06 rows=1 width=464)
> -> Sort (cost=128520.06..129493.57 rows=389404 width=464)
> Sort Key:
> (st_distance('0101000020AD100000F5BEF1B567835CC06A2E3718EA884940'::geometry,
> the_geom))
> -> Seq Scan on nrn_ab_8_0_roadseg (cost=0.00..126573.04
> rows=389404 width=464)
> (4 rows)
>
>
> Any suggests how to speed it up? Coming from MySQL, I'm brand-new to
> PostGIS (and Postgresql FWIW) and all the awesome spatial functions it has.
> I would think that maybe selecting a bounding box of rows, and then finding
> the one with the closest distance?
>
>
Yes exactly. That's how people do it now, in pre-PostGIS-2.0 era :-)

Make a search by bounding boxes, starting with some arbitraly selected
radius. Increase the radius until you have at least N=1 result found, than
sort these results by ST_Distance and select nearest neighbour.

PostGIS 2.0 solution: see
http://blog.opengeo.org/2011/09/28/indexed-nearest-neighbour-search-in-postgis/