Abeeujah commented on PR #298:
URL: https://github.com/apache/sedona-db/pull/298#issuecomment-3530211986
At a closer look, POSTGIS does actually support `GeometryCollection` as an
input to `ST_Boundary`, what it does is, it calculates the boundary for
individual geometries in the collection and returns them, here's a series of
tests
```sh
postgres=# SELECT ST_AsText(ST_Boundary('GEOMETRYCOLLECTION(LINESTRING(1 1,2
2),GEOMETRYCOLLECTION(POLYGON((3 3,4 4,5 5,3
3)),GEOMETRYCOLLECTION(LINESTRING(6 6,7 7),POLYGON((8 8,9 9,10 10,8 8)))))'));
st_astext
----------------------------------------------------------------------------------------------------------------
GEOMETRYCOLLECTION(MULTIPOINT((1 1),(2 2),(6 6),(7 7)),MULTILINESTRING((3
3,4 4,5 5,3 3),(8 8,9 9,10 10,8 8)))
(1 row)
postgres=# SELECT ST_AsText(ST_Boundary('GEOMETRYCOLLECTION(POLYGON ((0 0, 0
1, 1 1, 1 0, 0 0)), GEOMETRYCOLLECTION(LINESTRING(10 10, 10 20)))'));
st_astext
---------------------------------------------------------------------------------
GEOMETRYCOLLECTION(MULTIPOINT((10 10),(10 20)),LINESTRING(0 0,0 1,1 1,1 0,0
0))
(1 row)
postgres=# SELECT ST_AsText(ST_Boundary('GEOMETRYCOLLECTION(LINESTRING(10
10,20 20),GEOMETRYCOLLECTION(POLYGON((30 30,40 40,50 50,30
30)),GEOMETRYCOLLECTION(LINESTRING(60 60,70 70),LINESTRING(80 80,90 90))))'));
st_astext
---------------------------------------------------------------------------------------------------------------------
GEOMETRYCOLLECTION(MULTIPOINT((10 10),(20 20),(60 60),(70 70),(80 80),(90
90)),LINESTRING(30 30,40 40,50 50,30 30))
(1 row)
postgres=# SELECT ST_AsText(ST_Boundary('GEOMETRYCOLLECTION(POLYGON((1 1,2
2,3 3,1 1)),GEOMETRYCOLLECTION(LINESTRING(4 4,5
5),GEOMETRYCOLLECTION(POLYGON((6 6,7 7,8 8,6 6)),LINESTRING(9 9,10 10))))'));
st_astext
----------------------------------------------------------------------------------------------------------------
GEOMETRYCOLLECTION(MULTIPOINT((4 4),(5 5),(9 9),(10 10)),MULTILINESTRING((1
1,2 2,3 3,1 1),(6 6,7 7,8 8,6 6)))
(1 row)
postgres=# SELECT ST_AsText(ST_Boundary('GEOMETRYCOLLECTION(LINESTRING(1 1,1
10,10 10,10 1,1 1),GEOMETRYCOLLECTION(LINESTRING(2 2,2 9,9 9,9 2,2
2),GEOMETRYCOLLECTION(POLYGON((3 3,3 8,8 8,8 3,3 3)),POLYGON((4 4,4 7,7 7,7 4,4
4)))))'));
st_astext
--------------------------------------------------------------
MULTILINESTRING((3 3,3 8,8 8,8 3,3 3),(4 4,4 7,7 7,7 4,4 4))
(1 row)
postgres=# SELECT ST_AsText(ST_Boundary('GEOMETRYCOLLECTION(POLYGON((0 0,10
0,10 10,0 10,0 0)),GEOMETRYCOLLECTION(LINESTRING(1 1,9
9),GEOMETRYCOLLECTION(LINESTRING(1 9,9 1),POLYGON((2 2,8 2,8 8,2 8,2 2)))))'));
st_astext
--------------------------------------------------------------------------------------------------------------------------
GEOMETRYCOLLECTION(MULTIPOINT((1 1),(9 9),(1 9),(9 1)),MULTILINESTRING((0
0,10 0,10 10,0 10,0 0),(2 2,8 2,8 8,2 8,2 2)))
(1 row)
postgres=# SELECT
ST_AsText(ST_Boundary('GEOMETRYCOLLECTION(GEOMETRYCOLLECTION(GEOMETRYCOLLECTION(LINESTRING(1
2,3 4),POLYGON((5 6,7 8,9 10,5 6))),POLYGON((11 12,13 14,15 16,11
12))),LINESTRING(17 18,19 20))'));
st_astext
---------------------------------------------------------------------------------------------------------------------------
GEOMETRYCOLLECTION(MULTIPOINT((1 2),(3 4),(17 18),(19
20)),MULTILINESTRING((5 6,7 8,9 10,5 6),(11 12,13 14,15 16,11 12)))
(1 row)
postgres=#
```
This shaped the implementation, to support `GeometryCollection`, I had to
introduce the recursive call, to compute the boundary for the individual
geometries.
To be fully POSTGIS compliant, I had to group/aggregate the return the way
POSTGIS does
Might be a lot, but it's POSTGIS Compliant.
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