[postgis-devel] Proposal: ST_Overlay function

[Martin Davis]

Would it be useful to have a function that performs a full overlay of a set
of polygons?

The function would accept a single collection (or array) of polygons and
overlay them all.  It would return a geometry collection containing all the
resultants (but not including any gaps or holes in the input set).

This is similar to what is computed by the "node/polygonize" process
described here [1] - but with the improvement that areas which are not
covered by any input polygon are not included.  And it will be much faster,
and simpler to use.

[1] http://blog.cleverelephant.ca/2019/07/postgis-overlays.html

This is a geometry-only function, so as in the above process it is still
necessary to perform a Point-in-Polygon JOIN in order to attach parent
attribution to each resultant polygon. To do this the individual polygons
in the result have to be extracted using ST_Dump. (A full
geometry-attribute overlay is still highly desirable, but currently it's
not clear how this should/could operate in Postgres SQL.)

The function could be defined as an aggregate, in the same way that
ST_Union is an aggregate.

I suggest the name ST_Overlay for this function.  It could also be called
ST_Intersection, since in a sense it is the opposite of ST_Union. (This is
the name that R-sf uses for this process).  However, the analogy is not
exact, since unlike the binary ST_Intersection this function returns ALL
resultant areas, not just those which are common to every input geometry.

This could (and should) be defined for other geometry types too - although
it's not clear what the semantics should be.  It's tempting to say that
ST_Intersection on lines returns a fully noded and dissolved set of minimal
lines - but that is what ST_Union does now.  Perhaps ST_Union should change
to returning a *maximal* set of noded lines (i.e. merged node-to-node).

Thoughts and comments welcome.
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