#997: ST_Union performance degradation from GEOS 3.6.x -> 3.7.x
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Reporter: greenlaw | Owner: geos-devel@…
Type: defect | Status: new
Priority: major | Milestone:
Component: Default | Version: 3.7.0
Severity: Significant | Resolution:
Keywords: union |
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Comment (by mdavis):
I think the problem here is caused by a union operation encountering a
noding robustness error (TopologyException), on some pair of input
geometries. GEOS attempts to handle the error by invoking the heuristic
line snapping code. This is very slow for geoms with large numbers of
points.
The line snapping code has not changed for several versions. But there
have been some changes to low-level code for computing intersections.
This is likely producing a slightly different result, perhaps in the
buffer operation (ironically, the buffers may actually be more accurate
now, but because of that the linework of two geoms is more nearly
coincident - which is the classic cause of noding errors).
More thought is required to come up with an internal GEOS solution for
this. Some options are:
* Instead of the snapping heuristic use the buffer(0) hack (which in this
case also invokes some rounding to handle the noding issue, so does round
all output vertices somewhat)
* It might be possible to a priori decide if snapping will take "too long"
and only then use buffer(0)
* The new overlay code in the works does not use snapping (however, it
also requires some rounding)
Given that buffers are only an approximation, another workaround might be
to limit the precision of the computed buffers. This can be done now in
PostGIS by inserting a ST_SnapToGrid call after the buffer (although there
is a risk of introducing invalidity here - it would be better if ST_Buffer
allowed specifying a precision parameter, since internally it can
guarantee valid output).
JTS has the same problem (actually more obvious, since the slow snapping
occurs when unioning the two lower-right geometries from the set that Paul
posted)
--
Ticket URL: <https://trac.osgeo.org/geos/ticket/997#comment:7>
GEOS <http://trac.osgeo.org/geos>
GEOS (Geometry Engine - Open Source) is a C++ port of the Java Topology Suite
(JTS).
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