Re: auto_explain sample rate

On Fri, Mar 11, 2016 at 2:51 PM, Julien Rouhaud <julien(dot)rouhaud(at)dalibo(dot)com>
wrote:

> On 11/03/2016 11:45, Magnus Hagander wrote:
> >
> > Coming back to the previous discussions about random() - AFAICT this
> > patch will introduce the random() call always (in explain_ExecutorStart):
> >
> > +if (auto_explain_log_min_duration >= 0 && nesting_level == 0)
> > +current_query_sampled = (random() < auto_explain_sample_ratio *
> > +MAX_RANDOM_VALUE);
> >
> >
> > Not sure what the overhead is, but wouldn't it be better to include a
> > check for current_query_sampled>0 in the if part of that statement?
> > Regardless of performance, that feels cleaner to me. Or am I missing
> > something?
>
> You mean check for auto_explain_sample_ratio > 0 ?
>

I did, but I think what I should have meant is auto_explain_sample_ratio <
1.

> There would be a corner case if a query is sampled
> (current_query_sampled is true) and then auto_explain_sample_ratio is
> set to 0, all subsequent queries in this backend would be processed.
>

There would have to be an else block as well of course, that set it back.

> We could add a specific test for this case to spare a random() call, but
> I fear it'd be overkill. Maybe it's better to document that the good way
> to deactivate auto_explain is to set auto_explain.log_min_duration to -1.
>

I guess in the end it probably is - a general random() call is pretty cheap
compared to all the things we're doing. I think my mind was stuck in
crypto-random which can be more expensive :)

--
Magnus Hagander
Me: http://www.hagander.net/
Work: http://www.redpill-linpro.com/