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I am not sure what is the correct behavior here. The polygon consists of two separate parts (see the picture).
Here http://support.esri.com/en/knowledgebase/GISDictionary/term/polygon I see two definitions of 'polygon':
[data models] On a map, a closed shape defined by a connected sequence of x,y coordinate pairs, where the first and last coordinate pair are the same and all other pairs are unique.
[ESRI software] In ArcGIS software, a shape defined by one or more rings, where a ring is a path that starts and ends at the same point. If a polygon has more than one ring, the rings may be separate from one another or they may nest inside one another, but they may not overlap.
The polygon in question fits the second definition. Also, PostGIS which is rather strict about misuse of different shapes, agrees to create it as a polygon.
But both PostGIS and MariaDB return ST_NumInteriorRings = 1 for it. I don't know why, obviously there are no interior rings in here. I could not however find a clear definition of how ST_NumInteriorRings should behave in this case.
MariaDB [test]> select ST_NumInteriorRings(ST_PolyFromText('POLYGON((0 0,10 0,10 10,0 10,0 0),(-1 -1,-5 -1,-5 -5,-1 -5,-1 -1))'));
+------------------------------------------------------------------------------------------------------------+
| ST_NumInteriorRings(ST_PolyFromText('POLYGON((0 0,10 0,10 10,0 10,0 0),(-1 -1,-5 -1,-5 -5,-1 -5,-1 -1))')) |
+------------------------------------------------------------------------------------------------------------+
| 1 |
+------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
pgis=# select ST_NumInteriorRings(ST_PolyFromText('POLYGON((0 0,10 0,10 10,0 10,0 0),(-1 -1,-5 -1,-5 -5,-1 -5,-1 -1))'));
st_numinteriorrings
---------------------
1
(1 row)
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MDEV-7509
Testing for GIS extensions
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I have documented this on the https://mariadb.com/kb/en/mariadb/st_numinteriorrings/ page, will also do so on the https://mariadb.com/kb/en/mariadb/polygon page
That contradiction was created by the OpenGIS standard.
Accordingly to it, the POLYGON should have exactly one ExteriorRing and all the other rings should
lie within that ExteriorRing and thus be the InteriorRings.
But practically it's often convenient to let polygons have several 'ExteriorRings' and that doesn't make calculations on
shapes any more complicated. So that would be a shame to deprive user that convenience just to follow the standard.
So i belive we shouldn't fix this issue - rather document it.