On Mon, Jul 12, 2010 at 07:46:30PM +0200, Pavel Stehule wrote:
> 2010/7/12 Josip Rodin <joy@xxxxxxxxxxxxxx>:
> > On Mon, Jul 12, 2010 at 04:38:48PM +0200, Pavel Stehule wrote:
> >> 2010/7/12 Josip Rodin <joy@xxxxxxxxxxxxxx>:
> >> > On Mon, Jul 12, 2010 at 02:06:43PM +0800, Craig Ringer wrote:
> >> >> Meh, personally I'll stick to the good old profiling methods "is it fast
> >> >> enough", "\timing", and "explain analyze".
> >> >
> >> > I agree. Some hint could be included in 'explain analyze' output, maybe just
> >> > to separate the timings for things that are well covered by the query plan
> >> > optimizer from those that aren't. I found this in a line like this:
> >>
> >> it is useles for functions - explain doesn't show lines of executed
> >> functions. Can you show some example of some more complex query.
> >
> > It doesn't have to show me any lines, but it could tell me which part of
> > the query is actually being optimized, and OTOH which part is simply being
> > executed N times unconditionally because it's a function that is marked as
> > volatile. That alone would be a reasonable improvement.
>
> this is different kinds of problems. You can have a very slow a
> immutable function or very fast volatile function. And with wrong
> function design your functions can be a 10 times slower. yeah - you
> can multiply it via wrong or good design with wrong or good stability
> flag.
Well, it was demonstrated previously that the domain of very fast volatile
plpgsql functions is inherently limited with their startup overhead, which
makes them inherently slow with small data sets (and/or nesting). Sure, this
can become relatively insignificant on very large data sets, but as long as
there is a reasonable chance that this slows down a query by an order of
magnitude, IMHO it would be better to note it than to ignore it.
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