K-Nearest Neighbor Query in PostGIS
Solution 1
Just a few thoughts on your problem:
st_distance as well as st_area are not able to use indices. This is because both functions can not be reduced to questions like "Is a within b?" or "Do a and b overlap?". Even more concrete: GIST-indices can only operate on the bounding boxes of two objects.
For more information on this you just could look in the postgis manual, which states an example with st_distance and how the query could be improved to perform better.
However, this does not solve your k-nearest-neighbour-problem. For that, right now I do not have a good idea how to improve the performance of the query. The only chance I see would be assuming that the k nearest neighbors are always in a distance of below x meters. Then you could use a similar approach as done in the postgis manual.
Your second query could be speeded up a bit. Currently, you compute the area for each object in table 1 as often as table has rows - the strategy is first to join the data and then select based on that function. You could reduce the count of area computations significantly be precomputing the area:
WITH polygonareas AS (
SELECT gid, the_geom, st_area(the_geom) AS area
FROM polygons
)
SELECT g1.gid, g2.gid
FROM polygonareas as g1 , polygonareas as g2
WHERE g1.area > g2.area;
Your third query can be significantly optimized using bounding boxes: When the bounding boxes of two objects do not overlap, there is no way the objects do. This allows the usage of a given index and thus a huge performance gain.
Solution 2
Since late September 2011, PostGIS has supported indexed nearest neighbor queries via a special operator(s) usable in the ORDER BY clause:
SELECT name, gid
FROM geonames
ORDER BY geom <-> st_setsrid(st_makepoint(-90,40),4326)
LIMIT 10;
...will return the 10 objects whose geom
is nearest -90,40
in a scalable way. A few more details (options and caveats) are in that announcement post and use of the <-> and the <#> operators is also now documented in the official PostGIS 2.0 reference. (The main difference between the two is that <->
compares the shape centroids and <#>
compares their boundaries — no difference for points, other shapes choose what is appropriate for your queries.)
Solution 3
You can do it with KNN index and lateral join.
SELECT v.gid, v2.gid,st_distance(v.the_geom, v2.the_geom)
FROM geonames v,
lateral(select *
from geonames v2
where v2.id<>v.id
ORDER BY v.the_geom <-> v2.the_geom LIMIT 10) v2
where v.gid in (...) - or other filtering condition
Solution 4
What you may need is the KNN index which is hopefully available soon in PostGIS 2.x and PostgreSQL 9.1: See http://blog.opengeo.org/tag/knn/
Abhishek Sagar
i am 28 , Mtech in computer science at IIT Bombay , mumbai , india. I have joined Brocade Communications at bangalore in july 2012. I have done my Mtech thesis on Distributed computation on spatial data. My skype : abhishek004hbti email : [email protected] regards my friends .
Updated on June 19, 2022Comments
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Abhishek Sagar almost 2 years
I am using the following Nearest Neighbor Query in PostGIS :
SELECT g1.gid g2.gid FROM points as g1, polygons g2 WHERE g1.gid <> g2.gid ORDER BY g1.gid, ST_Distance(g1.the_geom,g2.the_geom) LIMIT k;
Now, that I have created indexes on the_geom as well as gid column on both the tables, this query is taking much more time than other spatial queries involving spatial joins b/w two tables.
Is there any better way to find K-nearest neighbors? I am using PostGIS.
And, another query which is taking a unusually long time despite creating indexes on geometry column is:
select g1.gid , g2.gid from polygons as g1 , polygons as g2 where st_area(g1.the_geom) > st_area(g2.the_geom) ;
I believe, these queries arent benefited by gist indexes, but why?
Whereas this query:
select a.polyid , sum(length(b.the_geom)) from polygon as a , roads as b where st_intersects(a.the_geom , b.the_geom);
returns result after some time despite involving "roads" table which is much bigger than polygons or points table and also involve more complex spatial operators.
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John Powell about 10 yearsA major caveat of these two operators, as it says on the linked postgis reference pages, is that the spatial index will only kick in if one of the geometries is a constant, as in your st_makepoint in the example. This means you can't use these operators with efficient index usage to answer the OP question which involves finding all geometries A near some other set of geometries B.
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natevw about 10 yearsAh, good point. Thanks for raising it. So is @Stefan's answer the "correct" one then, just needing a bit more detail and updated link(s)?