Обсуждение: select count(*) from pg_stat_activity in V8.0.0

Coming off of 7.4.3 I've always used

select count(*) from pg_stat_activity

through the ODBC driver to obtain the number of connections to the db.  I've
found that with v8.0 Beta 1 that this at times has returned a value of 0
(when there is obviously still a connection).  It's not repeatable, and
occurs sporadically.  Does anyone have any reason as to why this would be
isolated to V8?  Is this an acceptable means of obtaining the number of
connection to the database in V8 (via ODBC).

Thanks so much,
Peter

Anony Mous wrote:
> Coming off of 7.4.3 I've always used
>
> select count(*) from pg_stat_activity
>
> through the ODBC driver to obtain the number of connections to the db.  I've
> found that with v8.0 Beta 1 that this at times has returned a value of 0
> (when there is obviously still a connection).  It's not repeatable, and
> occurs sporadically.  Does anyone have any reason as to why this would be
> isolated to V8?  Is this an acceptable means of obtaining the number of
> connection to the database in V8 (via ODBC).

Do you have multiple live connections? My understanding of
pg_stat_activity is that it can lag slightly behind the current state of
the system. If the only connection that is active is the one doing the
counting you could be checking before the table has been updated.

--
   Richard Huxton
   Archonet Ltd

Richard Huxton <dev@archonet.com> writes:
> Anony Mous wrote:
>> select count(*) from pg_stat_activity

> Do you have multiple live connections? My understanding of
> pg_stat_activity is that it can lag slightly behind the current state of
> the system.

If memory serves, it's up to 500 msec behind.  So this technique has
never been 100% accurate.  I'm not sure why A.Mous failed to observe the
inaccuracy before ...

            regards, tom lane

Thanks, Richard.

I've never seen this behaviour before in 7.4.3 and indeed it is the only
connection to the back end at the time when the count is occurring. However,
it would have had the connection for at least 30 seconds before requesting a
count.

Is there a better method of obtaining the number of connections to the
backend?

Thanks
Peter

-----Original Message-----
From: Richard Huxton [mailto:dev@archonet.com]
Sent: August 19, 2004 2:21 AM
To: Anony Mous
Cc: pgsql-general@postgresql.org
Subject: Re: [GENERAL] select count(*) from pg_stat_activity in V8.0.0

Anony Mous wrote:
> Coming off of 7.4.3 I've always used
>
> select count(*) from pg_stat_activity
>
> through the ODBC driver to obtain the number of connections to the db.
I've
> found that with v8.0 Beta 1 that this at times has returned a value of 0
> (when there is obviously still a connection).  It's not repeatable, and
> occurs sporadically.  Does anyone have any reason as to why this would be
> isolated to V8?  Is this an acceptable means of obtaining the number of
> connection to the database in V8 (via ODBC).

Do you have multiple live connections? My understanding of
pg_stat_activity is that it can lag slightly behind the current state of
the system. If the only connection that is active is the one doing the
counting you could be checking before the table has been updated.

--
   Richard Huxton
   Archonet Ltd

Anony Mous wrote:
> Thanks, Richard.
>
> I've never seen this behaviour before in 7.4.3 and indeed it is the only
> connection to the back end at the time when the count is occurring. However,
> it would have had the connection for at least 30 seconds before requesting a
> count.
>
> Is there a better method of obtaining the number of connections to the
> backend?

Other than "ps" - no I don't think so. The 30 seconds doesn't sound
right, Tom reckoned 0.5sec which sounds plausible. Might this be the
first query you've sent on this connection?

I'm wondering if the ODBC connection isn't actually established until
you run the first query? It doesn't sound right to me, but I don't have
any of my ODBC apps to hand at the moment to check.

Could you:
1. Connect
2. SELECT 1; (or similar)
3. Sleep from 10msec => 1sec
4. SELECT * FROM pg_stat_activity;

That way we could disable step 2 and tinker with the delay in step 3 to
see what happens.

--
   Richard Huxton
   Archonet Ltd

Thanks, Richard.

I've never seen this behaviour before in 7.4.3 and indeed it is the only
connection to the back end at the time when the count is occurring.
However, it would have had the connection for at least 30 seconds before
requesting a count.

Is there a better method of obtaining the number of connections to the
backend?

Thanks
Peter

-----Original Message-----
From: Richard Huxton [mailto:dev@archonet.com]
Sent: August 19, 2004 2:21 AM
To: Anony Mous
Cc: pgsql-general@postgresql.org
Subject: Re: [GENERAL] select count(*) from pg_stat_activity in V8.0.0

Anony Mous wrote:
> Coming off of 7.4.3 I've always used
>
> select count(*) from pg_stat_activity
>
> through the ODBC driver to obtain the number of connections to the db.
I've
> found that with v8.0 Beta 1 that this at times has returned a value of 0
> (when there is obviously still a connection).  It's not repeatable, and
> occurs sporadically.  Does anyone have any reason as to why this would be
> isolated to V8?  Is this an acceptable means of obtaining the number of
> connection to the database in V8 (via ODBC).

Do you have multiple live connections? My understanding of
pg_stat_activity is that it can lag slightly behind the current state of
the system. If the only connection that is active is the one doing the
counting you could be checking before the table has been updated.

--
   Richard Huxton
   Archonet Ltd