[postgis-users] need a help

Obe, Regina robe.dnd at cityofboston.gov
Tue Nov 18 04:48:09 PST 2008

Try instead
SELECT r.name
 FROM (SELECT ST_Boundary(the_geom) As b_geom 
	FROM state WHERE  name ~* 'Nana') As s 
INNER JOIN road r ON ST_DWithin(s.b_geom, r.the_geom, 0.001)

1) Buffer is a relatively slow operation in PostGIS.  Faster to do a
distance within check.  
2) As I assume your state is a polygon so if you 
want to find out what runs along the boundary, you want to convert it to
a line string.  Checking against the polygon will give you 
everything inside the state which could be a lot of roads and I don't
think what you want.

3) The above subselect is probably not necessary and in theory shouldn't
make a difference, but writing a subquery  tends to guarantee to force
it to resolve subquery first.  It shouldn't make a difference actually.
I just do it out of habit.

Hope that helps,


From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
Nicholas I
Sent: Tuesday, November 18, 2008 1:36 AM
To: postgis-users at postgis.refractions.net
Subject: [postgis-users] need a help


The objective of the query is to find the roads that run along the state
boundry. I have tried and written a query, can somebody give me
suggestion on it. this below query runs but system hangs. i don't know
what is wrong.

select s.name from state s, road r where
st_intersects(ST_Buffer(s.the_geom, 0.001),r.the_geom) and s.name ~*

-Nicholas I

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