ST_SimplifyPreserveTopology doesn't always preserve topology with inner holes

[pnorman]

I have a geometry that has an inner hole. After applying ST_SimplifyPreserveTopology the hole lies outside the shell, rendering the polygon invalid.

The image in grey is the section of the polygon before simplification, the red overlay is after ST_SimplifyPreserveTopology(geom,40) has been applied.

The WKB is below

`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`

[mdavis]

This is a known bug in the original JTS implementation (see ​Javadoc ).

As mentioned there, the workaround is to simply remove holes which lie outside the polygon shell. But of course this needs to be implemented in GEOS (and JTS too ideally). There should be a GEOS ticket created for this.

[pramsey]

Closing this ticket in favour of a ticket in GEOS ​https://trac.osgeo.org/geos/ticket/967