Re: Allow auto_explain to log plans before queries are executed

On Thu, 27 Feb 2020 06:27:24 +0100
Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:

> čt 27. 2. 2020 v 6:16 odesílatel Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>
> napsal:
>
> > Hello.
> >
> > At Thu, 27 Feb 2020 10:18:16 +0900, Yugo NAGATA <nagata(at)sraoss(dot)co(dot)jp>
> > wrote in
> > > On Wed, 26 Feb 2020 18:51:21 +0100
> > > Julien Rouhaud <rjuju123(at)gmail(dot)com> wrote:
> > >
> > > > On Thu, Feb 27, 2020 at 02:35:18AM +0900, Yugo NAGATA wrote:
> > > > > Hi,
> > > > >
> > > > > Attached is a patch for allowing auto_explain to log plans before
> > > > > queries are executed.
> > > > >
> > > > > Currently, auto_explain logs plans only after query executions,
> > > > > so if a query gets stuck its plan could not be logged. If we can
> > > > > know plans of stuck queries, we may get some hints to resolve the
> > > > > stuck. This is useful when you are testing and debugging your
> > > > > application whose queries get stuck in some situations.
> > > >
> > > > Indeed that could be useful.
> > >
> > > > > This patch adds new option log_before_query to auto_explain.
> > > >
> > > > Maybe "log_before_execution" would be better?
> > >
> > > Thanks! This seems better also to me.
> > >
> > > >
> > > > > Setting auto_explain.log_before_query option logs all plans before
> > > > > queries are executed regardless of auto_explain.log_min_duration
> > > > > unless this is set -1 to disable logging. If log_before_query is
> > > > > enabled, only duration time is logged after query execution as in
> > > > > the case of when both log_statement and log_min_duration_statement
> > > > > are enabled.
> > > >
> > > > I'm not sure about this behavior. The final explain plan is needed at
> > least if
> > > > log_analyze, log_buffers or log_timing are enabled.
> > >
> > > In the current patch, log_before_query (will be log_before_execution)
> > > has no effect if log_analyze is enabled in order to avoid to log the
> > > same plans twice. Instead, is it better to log the plan always twice,
> > > before and after the execution, if log_before_query is enabled
> > > regardless of log_min_duration or log_analyze?
> >
> > Honestly, I don't think showing plans for all queries is useful
> > behavior.
> >
> > If you allow the stuck query to be canceled, showing plan in
> > PG_FINALLY() block in explain_ExecutorRun would work, which look like
> > this.
> >
> > explain_ExecutorRun()
> > {
> > ...
> > PG_TRY();
> > {
> > ...
> > else
> > starndard_ExecutorRun();
> > nesting_level--;
> > }
> > PG_CATCH();
> > {
> > nesting_level--;
> >
> > if (auto_explain_log_failed_plan &&
> > <maybe the time elapsed from start exceeds min_duration>)
> > {
> > 'show the plan'
> > }
> > }
> > }
> >
> > regards.
> >
>
> It can work - but still it is not good enough solution. We need "query
> debugger" that allows to get some query execution metrics online.
>
> There was a problem with memory management for passing plans between
> processes. Can we used temp files instead shared memory?

I think "query debugger" feature you proposed is out of scope of
auto_explain module. I also think the feature to analyze running
query online is great, but we will need another discussion on a new
module or eature for it.

Regards,
Yugo Nagata

--
Yugo NAGATA <nagata(at)sraoss(dot)co(dot)jp>