Why does ST_MakeValid() strip SRID from already-defined geometries? - postgresql

Question

I'm attempting to use the PostGIS ST_MakeValid() function on a series of mostly-concentric isodistance multipolygons . . .
. . . which are defined with clear geometry type and SRID (and while they may not be perfectly valid, are still valid enough to render in QGIS as shown above):
trade=# \d tmp1
Table "public.tmp1"
Column | Type | Collation | Nullable | Default
-----------+-----------------------------+-----------+----------+---------
the_geom | geometry(MultiPolygon,4326) | | |
Unfortunately, the ST_MakeValid() function is stripping both SRID and geometry type when I use it to create a new table:
trade=# CREATE TABLE tmp2 AS (SELECT ST_MakeValid(the_geom) AS the_geom_valid FROM tmp1);
SELECT 25
trade=# \d tmp2
Table "public.tmp2"
Column | Type | Collation | Nullable | Default
----------------+----------+-----------+----------+---------
the_geom_valid | geometry | | |
. . . and ST_SetSRID() cannot resolve it, either by creating a new table:
trade=# CREATE TABLE tmp3 AS (SELECT ST_SetSRID(the_geom_valid,4326) AS the_geom_srid FROM tmp2);
SELECT 25
trade=# \d tmp3
Table "public.tmp3"
Column | Type | Collation | Nullable | Default
---------------+----------+-----------+----------+---------
the_geom_srid | geometry | | |
. . . or by nesting functions:
trade=# CREATE TABLE tmp4 AS (SELECT ST_SetSRID(ST_MakeValid(the_geom),4326) AS the_geom_all FROM tmp1);
SELECT 25
trade=# \d tmp4
Table "public.tmp4"
Column | Type | Collation | Nullable | Default
--------------+----------+-----------+----------+---------
the_geom_all | geometry | | |
. . . or even by using everyone's favorite ST_MakeValid() semi-substitute, ST_Buffer():
trade=# CREATE TABLE tmp5 AS (SELECT ST_Buffer(the_geom,0) AS the_geom_buffer FROM tmp1);
SELECT 25
trade=# \d tmp5;
Table "public.tmp5"
Column | Type | Collation | Nullable | Default
-----------------+----------+-----------+----------+---------
the_geom_buffer | geometry | | |
I can't find any documentation that suggests this is expected behavior when using ST_MakeValid() - how can I create valid geometries without losing geom type and SRID?

Answer 1

The SRID is being preserved. For instance, try this:
SELECT st_srid(the_geom_buffer) FROM tmp2 LIMIT 1;
and you should see something like:
┌─────────┐
│ st_srid │
├─────────┤
│ 4326 │
└─────────┘
The reason for this is that ST_MakeValid() can return any of a number of types, but can't know what they'll be ahead of time. From the documentation:
In case of full or partial dimensional collapses, the output geometry may be a collection of lower-to-equal dimension geometries or a geometry of lower dimension.
So the only option is to return a generic geometry.
However, if you are confident that won't happen, you can force it using a cast (or an ALTER statement post hoc):
CREATE TABLE tmp2 AS
SELECT
ST_MakeValid(the_geom)::geometry(MultiPolygon, 4326) AS the_geom_valid
FROM tmp1;