Re: Cannot terminate backend

On Fri, 2023-03-31 at 13:46 +0000, Arnaud Lesauvage wrote:
> 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

The most likely explanation is that one of the PostGIS functions runs for
a long time without checking CHECK_FOR_INTERRUPTS().
That would be a PostGIS bug. Try to construct a reproducible test case
that you can share!

Perhaps this trick can help:
https://www.cybertec-postgresql.com/en/cancel-hanging-postgresql-query/

Yours,
Laurenz Albe