weird long time query - Mailing list pgsql-performance

SELECT
    procpid,  
    start,  
    now() - start AS lap,  
    current_query  
FROM
    (SELECT
        backendid,  
        pg_stat_get_backend_pid(S.backendid) AS procpid,  
        pg_stat_get_backend_activity_start(S.backendid) AS start,  
        pg_stat_get_backend_activity(S.backendid) AS current_query  
    FROM
        (SELECT pg_stat_get_backend_idset() AS backendid) AS S  
    ) AS S  
WHERE
    current_query <> '<IDLE>'
ORDER BY
    lap DESC;

Then, I found a SQL that has run for some days (and still running):

procpid       | 32638
start         | 2019-11-25 16:29:29.529318+08
lap           | 21 days 18:24:54.707369
current_query | DEALLOCATE pdo_stmt_00000388

I tried to kill it with: SELECT pg_cancel_backend(32638) but it takes no effects.

What's this query and what shall I do for it?

Best Wishes

Kaijiang