[postgis-users] ST_ClusterDBSCAN: is it deterministic?

[Giuseppe Broccolo]

Hi Daniel,

Il giorno ven 22 gen 2021 alle ore 18:07 Daniel Baston <dbaston at gmail.com>
ha scritto:

> It should be deterministic for most real data if the inputs are ordered
> consistently, using the OVER() clause as you suggest. It's possible that
> there may be a contrived situation involving duplicates in the input where
> a result would be different (as GEOS STRtree is using std::sort instead of
> std::stable_sort), but I'm not sure. Also, there are sometimes multiple
> possible clusterings that satisfy the DBSCAN algorithm, so it is expected
> that the results may differ from different implementations or different
> orderings of the same input.
>

Thank you for the answer. I think I'll try to define the partition
with the ORDER
BY geom clause in order to check if I can obtain more determinism. If I
correctly understood, the ORDER BY should add a further step with
preordering the geometries using an Hilbert curve. Of course, this would
impact the overall duration of the query.

Giuseppe.
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