[postgis-users] comparison of nearest neighbour solutions (st_dwithin() vs pgis_fn_nn() ) ?

[Obe, Regina]

Smith,
 
Actually I wrote that function and have to update it since its flawed
when dealing with non-point things and was also written pre-existence of
st_dwithin.
 
The difference is that pgis_fn_nn() is designed for returning multiple
near neighbors per record so comes in handy when you have a bulk of
records
 
e.g. 
for each street find the 5 closes houses
 
so if you had a set of 10 records of streets you could get up to 50
records returned back.
 
If you only need the closest record per record - using st_dwithin in
conjunction with distinct on is actually much faster -  take a look at
this example - the one involving DISTINCT ON
 
http://www.bostongis.com/PrinterFriendly.aspx?content_name=postgis_neare
st_neighbor
 
Hope that helps,
Regina

________________________________

From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
Smith Roman
Sent: Thursday, October 16, 2008 5:19 AM
To: Users Postgis
Subject: [postgis-users] comparison of nearest neighbour solutions
(st_dwithin() vs pgis_fn_nn() ) ?


Hi,
 
Thanks for the links to the postgis wiki on nearest neighbour. I have
postgis 1.3.3 so the use of the st_dwithin() function will work fine for
me because its very simple to use. But i have one more question.
 
 Whats the difference between the use of the st_dwithin() function and
the pgis_fn_nn() function ? I obtained pgis_fn_nn()  from the boston GIS
site
-http://www.bostongis.com/PrinterFriendly.aspx?content_name=postgis_near
est_neighbor_generic 
 
Is one more accurate than the other ?
 
Thanks,
 
Smith.



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