Thread: investigating slow queries through pg_stat_activity

investigating slow queries through pg_stat_activity

From
Dan Harris
Date:
I've got some queries generated by my application that will, for some
reason, run forever until I kill the pid.  Yet, when I run the
queries manually to check them out, they usually work fine.  To get
more information about these queries, I'm writing a utility to take
snapshots of pg_stat_activity every 5 minutes.  If it finds a query
that runs for longer than 15 minutes, it will trap the query so I can
run 'explain analyze' on it and see where the weakness is.

However, the problem I have is that pg_stat_activity only returns the
first n (255?) characters of the SQL as "current_query", so it gets
chopped off at the end.  I would very much like to find out how I can
get the *entire* query that is active.  Is this possible?

Also, I'm sure some people will respond with "turn on query
logging".. I've explored that option and the formatting of the log
file and the fact that EVERY query is logged is not what I'm after
for this project.  The "infinite-running" queries are unpredictable
and may only happen once a week.  Logging 24/7 in anticipation of one
of these occurrences is not something I'd like to do.

Thanks,

Dan Harris

Re: investigating slow queries through

From
Jacques Caron
Date:
Hi,

At 19:55 20/06/2005, Dan Harris wrote:
>Also, I'm sure some people will respond with "turn on query
>logging".. I've explored that option and the formatting of the log
>file and the fact that EVERY query is logged is not what I'm after
>for this project.

You can log just those queries that take "a little bit too much time". See
log_min_duration_statement in postgresql.conf. Set it really high, and
you'll only get those queries you're after.

Jacques.



Re: investigating slow queries through pg_stat_activity

From
Tom Lane
Date:
Dan Harris <fbsd@drivefaster.net> writes:
> However, the problem I have is that pg_stat_activity only returns the
> first n (255?) characters of the SQL as "current_query", so it gets
> chopped off at the end.  I would very much like to find out how I can
> get the *entire* query that is active.  Is this possible?

I think the limit is ~1000 characters in 8.0 and later.  However, you
can't realistically have "unlimited" because of constraints of the stats
messaging mechanism.

            regards, tom lane

Re: investigating slow queries through pg_stat_activity

From
George Essig
Date:
On 6/20/05, Dan Harris <fbsd@drivefaster.net> wrote:
> Also, I'm sure some people will respond with "turn on query
> logging".. I've explored that option and the formatting of the log
> file and the fact that EVERY query is logged is not what I'm after
> for this project.

You don't have to log every query.  You can set
log_min_duration_statement in postgresql.conf to log only the queries
that exceed a certain amount of time.

From the manual at
http://www.postgresql.org/docs/8.0/static/runtime-config.html:

log_min_duration_statement (integer)

    Sets a minimum statement execution time (in milliseconds) that
causes a statement to be logged. All SQL statements that run for the
time specified or longer will be logged with their duration. Setting
this to zero will print all queries and their durations. Minus-one
(the default) disables the feature. For example, if you set it to 250
then all SQL statements that run 250ms or longer will be logged.
Enabling this option can be useful in tracking down unoptimized
queries in your applications. Only superusers can change this setting.

George Essig

Re: investigating slow queries through pg_stat_activity

From
Enrico Weigelt
Date:
* Dan Harris <fbsd@drivefaster.net> wrote:

Hi,

> I've got some queries generated by my application that will, for some
> reason, run forever until I kill the pid.  Yet, when I run the
> queries manually to check them out, they usually work fine.

If you can change your application, you could try to encapsulate the
queries into views - this makes logging and tracking down problems
much easier.


cu
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