weird long time query

I'm using postgres 9.4.17 on centos 7.
I check the running queries with the following SQL:
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