Re: Common slow query reasons - help with a special log

On 10.12.2011 23:40, Daniel Cristian Cruz wrote:
> At work we have a 24 cores server, with a load average around 2.5.

A single query is processes by a single CPU, so even if the system is
not busy a single query may hit CPU bottleneck. The real issue is the
instrumentation overhead - timing etc. On some systems (with slow
gettimeofday) this may be a significant problem as the query hits the
CPU boundary sooner.

> I don't know yet if a system which use some unused CPU to minimize the
> load of a bad query identified early is bad or worse.

Not really, due to the "single query / single CPU" rule.

> Indeed, I don't know if my boss would let me test this at production
> too, but it could be good to know how things work in "auto-pilot" mode.

What I was pointing out is that you probably should not enable loggin
"explain analyze" output by "auto_explain.log_analyze = true". There are
three levels of detail:

1) basic, just log_min_duration_statement

2) auto_explain, without 'analyze' - just explain plain

3) auto_explain, with 'analyze' - explain plan with actual values

Levels (1) and (2) are quite safe (unless the minimum execution time is
too low).

Tomas

>
> 2011/12/10 Tomas Vondra <tv(at)fuzzy(dot)cz <mailto:tv(at)fuzzy(dot)cz>>
>
> There's auto_explain contrib module that does exactly what you're asking
> for. Anyway, explain analyze is quite expensive - think twice before
> enabling that on production server where you already have performance
> issues.
>
> Tomas
>
> On 10.12.2011 17:52, Daniel Cristian Cruz wrote:
> > Hi all,
> >
> > I'm trying to figure out some common slow queries running on the
> server,
> > by analyzing the slow queries log.
> >
> > I found debug_print_parse, debug_print_rewritten, debug_print_plan,
> > which are too much verbose and logs all queries.
> >
> > I was thinking in something like a simple explain analyze just for
> > queries logged with log_min_duration_statement with the query too.
> >
> > Is there a way to configure PostgreSQL to get this kind of
> information,
> > maybe I'm missing something? Is it too hard to hack into sources
> and do
> > it by hand? I never touched PostgreSQL sources.
> >
> > I'm thinking to write a paper that needs this information for my
> > postgraduate course. The focus of my work will be the log data, not
> > PostgreSQL itself. If I succeed, maybe it can be a tool to help
> all of us.
> >
> > Thank you,
> > --
> > Daniel Cristian Cruz
> > クルズ クリスチアン ダニエル
>
>
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> --
> Daniel Cristian Cruz
> クルズ クリスチアン ダニエル