[postgis-users] st_union

[paul.malm at lfv.se]

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-bounces at 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 <strk at kbt.io<mailto:strk at kbt.io>> wrote:
On Tue, Mar 03, 2020 at 10:12:05AM +0000, paul.malm at lfv.se<mailto:paul.malm at 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?
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/postgis-users/attachments/20200304/816ae67c/attachment.html>