Re: Best PostGIS function for finding the nearest line segment to a given point
| От | René Fournier |
|---|---|
| Тема | Re: Best PostGIS function for finding the nearest line segment to a given point |
| Дата | |
| Msg-id | 1F9D2E5C-C80D-4722-86FC-EE1A104E3CA5@gmail.com обсуждение исходный текст |
| Ответ на | Re: Best PostGIS function for finding the nearest line segment to a given point (Filip Rembiałkowski <plk.zuber@gmail.com>) |
| Ответы |
Re: Best PostGIS function for finding the nearest line
segment to a given point
(Filip Rembiałkowski <plk.zuber@gmail.com>)
|
| Список | pgsql-general |
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:
Does this appear optimal to you?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 distancegc3-# FROM nrn_ab_8_0_roadseggc3-# 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-05Alberta | 2004 | 2004 | 4 Street North-east | Calgary | 0.000663366090673065Alberta | 0 | 0 | 4 Street North-east | Calgary | 0.000667603774783403Alberta | 425 | 425 | 18 Avenue North-east | Calgary | 0.000835708003512673Alberta | 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 distancegc3-# FROM nrn_ab_8_0_roadseggc3-# 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)
Best regards,
René Fournier
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