| From: | Martijn van Oosterhout <kleptog(at)svana(dot)org> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Larry Rosenman <ler(at)lerctr(dot)org>, 'Alvaro Herrera' <alvherre(at)commandprompt(dot)com>, 'Simon Riggs' <simon(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgreSQL(dot)org |
| Subject: | Re: That EXPLAIN ANALYZE patch still needs work |
| Date: | 2006-06-09 15:18:00 |
| Message-ID: | 20060609151800.GB26418@svana.org |
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| Lists: | pgsql-hackers |
On Fri, Jun 09, 2006 at 10:00:20AM -0400, Tom Lane wrote:
> To tell you the truth, this information makes me even less pleased with
> the sampling-gettimeofday patch than I was before. If gettimeofday() in
> itself increases the runtime of a node by a factor of 10, then just
> trying to subtract off that time is no solution. There's too much
> impact on surrounding nodes, and too much roundoff error anyhow.
> I had thought we were applying an order-of-ten-percent correction by
> subtracting SampleOverhead, not an order-of-10x correction :-(
Eh? The whole point is to call gettimeofday() much less often. If you
call it 1000th as often, then the correction is only on the order of
one hundredth of the normal query time...
Subtracting SampleOverhead is only a correction on the order of a few
percent, it's the reduced calling of gettimeofday() that provides the
benefit.
Have a nice day,
--
Martijn van Oosterhout <kleptog(at)svana(dot)org> http://svana.org/kleptog/
> From each according to his ability. To each according to his ability to litigate.
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