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<p class=MsoNormal>Hi All,<o:p></o:p></p>
<p class=MsoNormal> 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/">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><o:p> </o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New"'>SELECT
name, gid<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New"'>FROM
geonames<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New"'>ORDER
BY geom <-> st_setsrid(st_makepoint(-90,40),4326)<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New"'>LIMIT
10;<o:p></o:p></span></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>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><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><o:p> </o:p></p>
<p class=MsoNormal>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><o:p> </o:p></p>
<p class=MsoNormal><span style='font-family:"Courier New"'>CREATE TABLE
n2180_560_height AS <o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Courier New"'>SELECT x, y, height
FROM <o:p></o:p></span></p>
<p class=MsoNormal><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<o:p></o:p></span></p>
<p class=MsoNormal><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<o:p></o:p></span></p>
<p class=MsoNormal><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)<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Courier New"'>ORDER BY veg.gid,
ST_Distance(veg.the_geom,ground.the_geom)) AS vegpoints;<o:p></o:p></span></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>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><o:p> </o:p></p>
<p class=MsoNormal>Thanks,<o:p></o:p></p>
<p class=MsoNormal>Best,<o:p></o:p></p>
<p class=MsoNormal>Steve<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
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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'>(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><a href="http://www.clemetparks.com/"><span style='font-size:9.0pt;
font-family:"Arial","sans-serif"'>clevelandmetroparks.com</span></a><o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
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