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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">For one script I’m buffering lines and joining those buffers, some overlapping.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">For another script I’m joining polygons that are cutted in sub maps, no overlapping.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">So there is both variants and both of those were faster with st_buffer.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">/Paul<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">Från:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> postgis-users [mailto:postgis-users-bounces@lists.osgeo.org]
<b>För </b>Martin Davis<br>
<b>Skickat:</b> den 3 mars 2020 15:38<br>
<b>Till:</b> PostGIS Users Discussion; Martin Davis<br>
<b>Ämne:</b> Re: [postgis-users] st_union<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Tue, Mar 3, 2020 at 2:31 AM Sandro Santilli <<a href="mailto:strk@kbt.io">strk@kbt.io</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">On Tue, Mar 03, 2020 at 10:12:05AM +0000, <a href="mailto:paul.malm@lfv.se" target="_blank">
paul.malm@lfv.se</a> wrote:<br>
> There was no difference between st_union() and st_unaryunion() when merging all adjacent polygons to larger polygons.<br>
> The st_buffer() method is 3.5 times faster.<br>
<br>
The reason why ST_Buffer is faster is because it builds the topology<br>
of the geometry only once, while ST_Union builds it for each pair<br>
of geometries taken in exam. The ST_UnaryUnion version simply chooses<br>
which pair of geometries to union in order for the operation to be<br>
as effective as possible (drop more edges).<br>
<br>
There was a reason why we decided NOT to use GEOS Buffer internally<br>
for UnaryUnion, probably had to do with robustness, but I forgot the<br>
details. Maybe Martin can help here ?<o:p></o:p></p>
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<p class="MsoNormal">Unioning by buffer is sometimes faster than Unary Union (Cascaded), and sometimes not. It all depends on the characteristics of the geometry being processed. If there are a lot of overlapping geometries then Cascaded Union is faster. <o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">So both techniques are now available, and users can choose which works better for their data.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Paul, what does your dataset look like? <o:p></o:p></p>
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