SRID lost in ST_UNION

[Erik Wienhold]

On 2024-10-15 12:14 +0200, Łukasz Szałek via postgis-users wrote:
> 1)
> PostgreSQL 15.1 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-44), 64-bit
> POSTGIS="3.3.2 4975da8" [EXTENSION] PGSQL="150" GEOS="3.11.1-CAPI-1.17.1" PROJ="7.2.1" LIBXML="2.9.1" LIBJSON="0.11"
> 
> Query:
> select ST_SRID(ST_GeomFromText('POLYGON ((4.9882543 50.4415584, 4.9882543 50.4415586, 4.9882545 50.4415586, 4.9882545 50.4415584, 4.9882543 50.4415584))',4326)), st_srid(ST_UNION(ARRAY[ST_GeomFromText('POLYGON ((4.9882543 50.4415584, 4.9882543 50.4415586, 4.9882545 50.4415586, 4.9882545 50.4415584, 4.9882543 50.4415584))',4326)]));
> st_srid | st_srid
> ---------+---------
>     4326 |    4326
> (1 row)
> 
> 
> 2)
> PostgreSQL 17.0 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 11.4.1 20231218 (Red Hat 11.4.1-3), 64-bit
> POSTGIS="3.3.7 a0c7967" [EXTENSION] PGSQL="170" GEOS="3.13.0-CAPI-1.19.0" PROJ="9.5.0" LIBXML="2.9.13" LIBJSON="0.14" LIBPROTOBUF="1.3.3" WAGYU="0.5.0 (Internal)"
> 
> Query:
> select ST_SRID(ST_GeomFromText('POLYGON ((4.9882543 50.4415584, 4.9882543 50.4415586, 4.9882545 50.4415586, 4.9882545 50.4415584, 4.9882543 50.4415584))',4326)), st_srid(ST_UNION(ARRAY[ST_GeomFromText('POLYGON ((4.9882543 50.4415584, 4.9882543 50.4415586, 4.9882545 50.4415586, 4.9882545 50.4415584, 4.9882543 50.4415584))',4326)]));
> st_srid | st_srid
> ---------+---------
>     4326 |       0
> 
> Is there a regression between those versions?

Yes, I see the same regression in 3.4.3 as well.  This is caused by [1]
which sets SRID=0 [2] because the SRID has not been determined at this
point.

[1] https://trac.osgeo.org/postgis/ticket/5677
[2] https://trac.osgeo.org/postgis/browser/git/postgis/lwgeom_geos.c?rev=f37390b9cc4b33e8a7ec15e0396112fca19391c9#L659

-- 
Erik