For one script I’m buffering lines and joining those buffers, some overlapping. For another script I’m joining polygons that are cutted in sub maps, no overlapping. So there is both variants and both of those were faster with st_buffer. /Paul
Från: postgis-users [mailto:postgis-users-boun...@lists.osgeo.org] För Martin Davis Skickat: den 3 mars 2020 15:38 Till: PostGIS Users Discussion; Martin Davis Ämne: Re: [postgis-users] st_union On Tue, Mar 3, 2020 at 2:31 AM Sandro Santilli <s...@kbt.io<mailto:s...@kbt.io>> wrote: On Tue, Mar 03, 2020 at 10:12:05AM +0000, paul.m...@lfv.se<mailto:paul.m...@lfv.se> wrote: > There was no difference between st_union() and st_unaryunion() when merging > all adjacent polygons to larger polygons. > The st_buffer() method is 3.5 times faster. The reason why ST_Buffer is faster is because it builds the topology of the geometry only once, while ST_Union builds it for each pair of geometries taken in exam. The ST_UnaryUnion version simply chooses which pair of geometries to union in order for the operation to be as effective as possible (drop more edges). There was a reason why we decided NOT to use GEOS Buffer internally for UnaryUnion, probably had to do with robustness, but I forgot the details. Maybe Martin can help here ? 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. So both techniques are now available, and users can choose which works better for their data. Paul, what does your dataset look like?
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