Thread: Fwd: weird long time query

On Wed, Dec 18, 2019 at 11:25:32AM +0800, Kaijiang Chen wrote:
> 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?
> 
> I think it is a bug since logically, this query should be gone.

It's not a bug. Most likely this backend is not doing anything.

You're using old way to check if backend is working - current_query <>
'<IDLE>';

Check: select * from pg_stat_activity where pid = 32638 

Most likely you'll see state = 'idle'

In such cases, query just shows last executed query, not currently
running one.

Also - WHY are you calling internal pg* functions directly, instead of
using pg_stat_activity view?

Best regards,

depesz

On Thu, Dec 19, 2019 at 12:14:26AM +0800, Kaijiang Chen wrote:
> Thanks!
> I learn the SQL from the web. pg views should be better.
> BTW, I got the similar result (still see that proc) with "select * from
> pg_stat_activity":
> 
> backend_start    | 2019-11-25 16:27:05.103901+08
> xact_start       |
> query_start      | 2019-11-25 16:29:29.529318+08
> state_change     | 2019-11-25 16:29:29.529344+08
> waiting          | f
> state            | idle
> backend_xid      |
> backend_xmin     |
> query            | DEALLOCATE pdo_stmt_00000388
> 
> Looks not very nice :-)

not sure what you mean by not nice.

As you can clearly see the backend is *NOT* running anything (state is
idle).

Value in "query" column is simply last query that it ran. It *finished*
running this query at 2019-11-25 16:29:29.529344+08.

So your app is keeping connection open. It's not Pg problem or a bug.

Best regards,

depesz

hubert depesz lubaczewski <depesz@depesz.com> writes:
> On Thu, Dec 19, 2019 at 12:14:26AM +0800, Kaijiang Chen wrote:
>> BTW, I got the similar result (still see that proc) with "select * from
>> pg_stat_activity":
>> ...
>> state            | idle
>> ...
>> query            | DEALLOCATE pdo_stmt_00000388
>> 
>> Looks not very nice :-)

> not sure what you mean by not nice.

That's a feature not a bug (and yes, the behavior is documented).
People requested that the view continue to display the last query
of an idle session.  IIRC, the main argument was that otherwise
it's hard to tell apart a bunch of idle sessions.

If you don't like it, you can always do something like

case when state = idle then null else query end

            regards, tom lane