parse error - invalid geometry even though ST_ISVALID returns true

Question

I have a function to return a reduced size version of the geometries in our database.

CREATE OR REPLACE FUNCTION fog.small_kml(
    shape geometry,
    max_length integer)
    RETURNS text
    LANGUAGE 'plpgsql'
AS $BODY$
 DECLARE kml text;
 DECLARE tolerance float;
BEGIN
    tolerance:=0.001;
    SELECT ST_ASKML(shape) INTO kml;

    WHILE LENGTH(kml) > max_length LOOP
        SELECT ST_ASKML(ST_SIMPLIFY(shape, tolerance)) INTO kml;
        tolerance = tolerance + 0.001;
    END LOOP;

    return kml;
END
 $BODY$;

when I run this on certain geometries I get get the result

ERROR:  parse error - invalid geometry
HINT:  "<M" <-- parse error at position 2 within geometry
SQL state: XX000

occurring due to the statement SELECT ST_ASKML(ST_SIMPLIFY(shape, tolerance)) INTO kml;. However, ST_ISVALID says that the geometries are all valid.

Is there anything I can do to force the geometries into a state where I can simplify them? The length of the KML is around 12 million characters for the largest, which is why I want to simplify (and why I can't paste the whole thing here).

Each one starts <MultiGeometry><Polygon><outerBoundaryIs><LinearRing><coordinates> and I've had no issues with them before other than being slow. It seems to be ST_SIMPLIFY only that has an issue.

Answer 1

Vince got it correct in the comments - simplification led to an invalid geometry and then it was the ST_AsKML function which threw the error.

I fixed it by changing that line to

SELECT ST_AsKML(ST_MakeValid(ST_Simplify(shape, tolerance))) INTO kml;