Cannot terminate backend

Hi all,

I have a long running query that I seem unable to either cancel or terminate.
What could be the reason for this, and what is the bet way to terminate this kind of query ?

The query is a CTE using postgis ST_ClusterDBSCAN function. The CTE returns approximately 150k rows.
The SQL is as follows :

EXPLAIN ANALYZE

WITH subq AS (

SELECT id, geom, ST_ClusterDBSCAN(geom, eps := 1000, minpoints := 1) OVER() AS cluster_id
FROM mytable
)

SELECT cluster_id, count(id), ST_Collect(geom)

FROM subq

GROUP BY cluster_id;

pg_stat_activity show no wait event.
pg_cancel_backend(mypid) returns true but the state does not change in pg_stat_activity.
pg_terminate_backend(mypid) yields the same result (as superuser)
Pg_stat_activity show no wait_event.

SELECT version();

PostgreSQL 14.5 (Ubuntu 14.5-1.pgdg20.04+1) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0, 64-bit

Any idea ? Thanks a lot !

Regards
Arnaud