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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Ah, I had hopes pinned on the idea that I just wasn’t
smart enough to figure it out, but it’s an inherent limitation. I
will be using your function though—that is a clean way to encapsulate the
functionality.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
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</v:shape><![endif]--><![if !vml]><img width=90 height=122
src="cid:image001.png@01CD3341.CF198760" align=left hspace=12
alt="http://www.clemetparks.com/images/esig/cmp-ms-90x122.png" v:shapes="cmp-ms-90x122.png"><![endif]><span
style='font-size:14.0pt;font-family:"Arial","sans-serif";color:#006C56'>Stephen
Mather<br>
</span><span style='font-size:11.0pt;font-family:"Arial","sans-serif";
color:#006C56'>Geographic Information Systems (GIS) Manager<br>
</span><span style='font-size:9.0pt;font-family:"Arial","sans-serif";
color:#006C56'>(216) 635-3243<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:9.0pt;font-family:"Arial","sans-serif";
color:#006C56'>svm@clevelandmetroparks.com<br>
</span><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><a href="http://www.clemetparks.com/"><span style='font-size:
9.0pt;font-family:"Arial","sans-serif";color:blue'>clevelandmetroparks.com</span></a><o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'>
<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span
style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>
postgis-users-bounces@postgis.refractions.net
[mailto:postgis-users-bounces@postgis.refractions.net] <b>On Behalf Of </b>Alexandre
Neto<br>
<b>Sent:</b> Wednesday, May 16, 2012 7:35 AM<br>
<b>To:</b> PostGIS Users Discussion<br>
<b>Subject:</b> Re: [postgis-users] PostGIS KNN best practices<o:p></o:p></span></p>
</div>
<p class=MsoNormal><o:p> </o:p></p>
<div>
<p class=MsoNormal>I have been around that question to.<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal><a
href="http://gis.stackexchange.com/questions/24456/nearest-neighbor-problem-in-postgis-2-0-using-gist-index-function">http://gis.stackexchange.com/questions/24456/nearest-neighbor-problem-in-postgis-2-0-using-gist-index-function</a> <o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>You have to do it in two steps, like is explained in the
operator <a
href="http://postgis.refractions.net/docs/geometry_distance_centroid.html">page</a>.
One faster step to reduce the candidates (by using <-> or <#>) and
second one to get the real distances with ST_Distance.<o:p></o:p></p>
</div>
<p class=MsoNormal><o:p> </o:p></p>
<div>
<p class=MsoNormal>The problem in finding the KNN for each row in a table is
the fact that the gist index <-> operator only works if one of the
geometries is constant. The workaround would be to create a SQL function to
apply to each of the rows using table.the_geom as a parameter.<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>Something like this:<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>----<o:p></o:p></p>
</div>
<div>
<div>
<p class=MsoNormal>CREATE OR REPLACE FUNCTION _enn2 (geometry) RETURNS double
precision AS $$<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>WITH index_query as<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>(SELECT ST_Distance($1,f.the_geom) as dist<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>FROM "grelha5m" As f<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>ORDER BY $1 <#> g1.the_geom limit 1000)<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>SELECT dist<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>FROM index_query<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>ORDER BY dist;<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>$$ LANGUAGE SQL;<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>---<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>and I call it like this: <o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>---<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>Select c.gid as gid, _enn2(c.the_geom) as enn<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>From cosn1 as c<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>Order by c.gid<o:p></o:p></p>
</div>
</div>
<div>
<p class=MsoNormal>---<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>In this case the function returned the smallest distance,
but you can choose any other column.<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>Hope it helps<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>Alexandre Neto<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>On Tue, May 15, 2012 at 5:37 PM, Stephen V. Mather <<a
href="mailto:svm@clevelandmetroparks.com" target="_blank">svm@clevelandmetroparks.com</a>>
wrote:<o:p></o:p></p>
<div>
<div>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Hi
All,<o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>
Pretty excited by the new operators <-> and <#>, but a bit confused
as to how to use them in a query. The two examples from P. Ramsey back in
late 2011 ( <a
href="http://blog.opengeo.org/2011/09/28/indexed-nearest-neighbour-search-in-postgis/"
target="_blank">http://blog.opengeo.org/2011/09/28/indexed-nearest-neighbour-search-in-postgis/</a>
) included doing a KNN on a single point to a cloud of points, i.e. <o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-size:10.0pt;font-family:"Courier New"'>SELECT name, gid</span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-size:10.0pt;font-family:"Courier New"'>FROM geonames</span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-size:10.0pt;font-family:"Courier New"'>ORDER BY geom <->
st_setsrid(st_makepoint(-90,40),4326)</span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-size:10.0pt;font-family:"Courier New"'>LIMIT 10;</span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>or
doing KNN on non-point different geometries, where the first neighbor by
<-> or <#> might not be truly the first i.e.<o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p>
<pre>with index_query as (<o:p></o:p></pre><pre> select<o:p></o:p></pre><pre> st_distance(geom, 'SRID=3005;POINT(1011102 450541)') as distance,<o:p></o:p></pre><pre> parcel_id, address<o:p></o:p></pre><pre> from parcels<o:p></o:p></pre><pre> order by geom <#> 'SRID=3005;POINT(1011102 450541)' limit 100<o:p></o:p></pre><pre>)<o:p></o:p></pre><pre>select * from index_query order by distance limit 10;<o:p></o:p></pre>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>So,
how would one grab the first nearest neighbor for all points in a
dataset? This is how I used to do it:<o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-family:"Courier New"'>CREATE TABLE n2180_560_height AS </span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-family:"Courier New"'>SELECT x, y, height FROM </span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-family:"Courier New"'>(SELECT DISTINCT ON(veg.gid) veg.gid as gid,
ground.gid as gid_ground, veg.x as x, veg.y as y, ground.z as z, veg.z -
ground.z as height, veg.the_geom as geometry, veg.class as class</span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-family:"Courier New"'>FROM (SELECT * FROM n2180_560 WHERE class =
5) As veg, (SELECT * FROM n2180_560 WHERE class = 2) As ground</span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-family:"Courier New"'>WHERE veg.class = 5 AND veg.gid <>
ground.gid AND ST_DWithin(veg.the_geom, ground.the_geom, 10)</span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-family:"Courier New"'>ORDER BY veg.gid,
ST_Distance(veg.the_geom,ground.the_geom)) AS vegpoints;</span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>ST_DWithin
prevents a full cross join, but is a sloppy way to do this, as it requires a
priori knowledge of the end cases. I’m hoping there’s a
subquery or some such magic that would allow me to use the distance operator to
a similar end… .<o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Thanks,<o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Best,<o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Steve<o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><!--[if gte vml 1]><v:shape
id="_x0000_s1026" type="#_x0000_t75" alt="http://www.clemetparks.com/images/esig/cmp-ms-90x122.png"
style='position:absolute;margin-left:0;margin-top:0;width:67.5pt;height:91.5pt;
z-index:1;mso-position-horizontal:left;mso-position-vertical-relative:line'
o:allowoverlap="f">
<v:imagedata src="cid:image001.png@01CD3341.CF198760" o:title="image001" />
<w:wrap type="square"/>
</v:shape><![endif]--><![if !vml]><img width=90 height=122
src="cid:image001.png@01CD3341.CF198760" align=left hspace=12
alt="http://www.clemetparks.com/images/esig/cmp-ms-90x122.png" v:shapes="_x0000_s1026"><![endif]><span
style='font-size:14.0pt;font-family:"Arial","sans-serif";color:#006C56'>Stephen
Mather<br>
</span><span style='font-family:"Arial","sans-serif";color:#006C56'>Geographic
Information Systems (GIS) Manager<br>
</span><span style='font-size:9.0pt;font-family:"Arial","sans-serif";
color:#006C56'><a href="tel:%28216%29%20635-3243" target="_blank">(216)
635-3243</a></span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-size:9.0pt;font-family:"Arial","sans-serif";color:#006C56'><a
href="mailto:svm@clevelandmetroparks.com" target="_blank">svm@clevelandmetroparks.com</a><br>
</span><a href="http://www.clemetparks.com/" target="_blank"><span
style='font-size:9.0pt;font-family:"Arial","sans-serif"'>clevelandmetroparks.com</span></a><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p>
</div>
</div>
<p class=MsoNormal style='margin-bottom:12.0pt'><br>
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<p class=MsoNormal><o:p> </o:p></p>
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