On Fri, Mar 11, 2016 at 2:51 PM, Julien Rouhaud <julien.rouh...@dalibo.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/
