[postgis-users] st_union

[Sandro Santilli]

On Tue, Mar 03, 2020 at 10:12:05AM +0000, 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 ?

--strk;