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<p class=MsoNormal>Hi All,<o:p></o:p></p>
<p class=MsoNormal> True
confession, I’m running PostGIS 1.3.5, so throw the tomatoes gently if A)
I’m doing a bad query, or B) I’m dredging up old news on (in)efficiency,
and I really should upgrade to 1.5.x, thank you very much… . Admittedly,
it is time for that, but hopefully that’s a separate issue.<o:p></o:p></p>
<p class=MsoNormal> I’m
trying to calculate the symmetrical difference on a pair of geometries, and my
query runs slowly:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal><span style='font-family:"Courier New"'> SELECT
ST_SymDifference(<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Courier New"'> ST_Collect(neww.the_geom),<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Courier New"'> ST_Collect(oldd.the_geom)
<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Courier New"'> )
AS the_geom<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Courier New"'> FROM
boundary neww, boundary_old oldd;<o:p></o:p></span></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal style='text-indent:.5in'>Now, if I were to be entirely
honest, I’d confess that this was my cludge of an original query:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal><span style='font-family:"Courier New"'> SELECT
ST_SymDifference(<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Courier New"'> ST_Union(ST_Buffer(neww.the_geom,
0)),<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Courier New"'> ST_Union(ST_Buffer(oldd.the_geom,
0)) <o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Courier New"'> )
AS the_geom<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Courier New"'> FROM
boundary neww, boundary_old oldd;<o:p></o:p></span></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>but who would admit to running a query like that, should
they know better. Not I.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>So, why is my query sooooo slooooow? Is it A), B), or
some yet to be known to me C). BTW, (now I’m just to being
provocative,) when I run a symmetrical difference in ArcGIS, it runs very
quickly… :).<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Best,<o:p></o:p></p>
<p class=MsoNormal>Thanks,<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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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-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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