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

Hi Filip,

On 2011-10-08, at 2:26 AM, Filip Rembiałkowski wrote:

> 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/

Thanks. Based on some further reading, this is what I came up with, in order to hopefully use the GiST index to greatest benefit:

gc3=# SELECT datasetnam, r_hnumf, r_hnuml, r_stname_c, r_placenam, ST_Distance(ST_GeomFromText('POINT(-114.053205 51.069644)',4269),the_geom) AS distance
gc3-# FROM nrn_ab_8_0_roadseg
gc3-# WHERE the_geom && SetSRID('BOX3D(-114.1 49.9,-113.9 51.1)'::box3d,4269)
gc3-# ORDER BY distance ASC LIMIT 5;
datasetnam | r_hnumf | r_hnuml | r_stname_c | r_placenam | distance
------------+---------+---------+----------------------+------------+----------------------
Alberta | 407 | 459 | 19 Avenue North-east | Calgary | 5.74515867479735e-05
Alberta | 2004 | 2004 | 4 Street North-east | Calgary | 0.000663366090673065
Alberta | 0 | 0 | 4 Street North-east | Calgary | 0.000667603774783403
Alberta | 425 | 425 | 18 Avenue North-east | Calgary | 0.000835708003512673
Alberta | 407 | 449 | 20 Avenue North-east | Calgary | 0.000981910679856406
(5 rows)

gc3=# EXPLAIN SELECT datasetnam, r_hnumf, r_hnuml, r_stname_c, r_placenam, ST_Distance(ST_GeomFromText('POINT(-114.053205 51.069644)',4269),the_geom) AS distance
gc3-# FROM nrn_ab_8_0_roadseg
gc3-# WHERE the_geom && SetSRID('BOX3D(-114.1 49.9,-113.9 51.1)'::box3d,4269)
gc3-# ORDER BY distance ASC LIMIT 5;
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=33632.15..33632.16 rows=5 width=480)
-> Sort (cost=33632.15..33693.00 rows=24341 width=480)
Sort Key: (st_distance('0101000020AD100000F5BEF1B567835CC06A2E3718EA884940'::geometry, the_geom))
-> Bitmap Heap Scan on nrn_ab_8_0_roadseg (cost=812.99..33227.85 rows=24341 width=480)
Recheck Cond: (the_geom && '0103000020AD10000001000000050000006666666666865CC03333333333F348406666666666865CC0CDCCCCCCCC8C49409A99999999795CC0CDCCCCCCCC8C49409A99999999795CC03333333333F348406666666666865CC03333333333F34840'::geometry)
-> Bitmap Index Scan on nrn_ab_8_0_roadseg_the_geom_gist (cost=0.00..806.91 rows=24341 width=0)
Index Cond: (the_geom && '0103000020AD10000001000000050000006666666666865CC03333333333F348406666666666865CC0CDCCCCCCCC8C49409A99999999795CC0CDCCCCCCCC8C49409A99999999795CC03333333333F348406666666666865CC03333333333F34840'::geometry)
(7 rows)

Does this appear optimal to you?

Best regards,
René Fournier