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Re: Possible copy error in explain example 3+ messages / 2 participants [nested] [flat]
* Re: Possible copy error in explain example @ 2005-04-12 04:34 Michael Fuhr <[email protected]> parent: Bruno Wolff III <[email protected]> 0 siblings, 1 reply; 3+ messages in thread From: Michael Fuhr @ 2005-04-12 04:34 UTC (permalink / raw) To: Bruno Wolff III <[email protected]>; pgsql-docs On Mon, Apr 11, 2005 at 11:40:01PM -0500, Bruno Wolff III wrote: > I was looking at: > http://candle.pha.pa.us/main/writings/pgsql/sgml/performance-tips.html > > And noticed in the first example that it is claimed the cost estimate > is 233 disk block reads. However, "233" doesn't appear in the explain > output shown. I think that "233" is supposed to match the "333" in > the explain output, but I am not 100% sure. This part? This is about as straightforward as it gets. If you do SELECT * FROM pg_class WHERE relname = 'tenk1'; you will find out that tenk1 has 233 disk pages and 10000 rows. So the cost is estimated at 233 page reads, defined as costing 1.0 apiece, plus 10000 * cpu_tuple_cost which is currently 0.01 (try SHOW cpu_tuple_cost). Doesn't that work out to (233 * 1.0) + (10000 * 0.01) = 233.0 + 100.0 = 333.0 or am I missing something? -- Michael Fuhr http://www.fuhr.org/~mfuhr/ ^ permalink raw reply [nested|flat] 3+ messages in thread
* Possible copy error in explain example @ 2005-04-12 04:40 Bruno Wolff III <[email protected]> 0 siblings, 1 reply; 3+ messages in thread From: Bruno Wolff III @ 2005-04-12 04:40 UTC (permalink / raw) To: pgsql-docs I was looking at: http://candle.pha.pa.us/main/writings/pgsql/sgml/performance-tips.html And noticed in the first example that it is claimed the cost estimate is 233 disk block reads. However, "233" doesn't appear in the explain output shown. I think that "233" is supposed to match the "333" in the explain output, but I am not 100% sure. ^ permalink raw reply [nested|flat] 3+ messages in thread
* Re: Possible copy error in explain example @ 2005-04-12 05:01 Bruno Wolff III <[email protected]> parent: Michael Fuhr <[email protected]> 0 siblings, 0 replies; 3+ messages in thread From: Bruno Wolff III @ 2005-04-12 05:01 UTC (permalink / raw) To: Michael Fuhr <[email protected]>; +Cc: pgsql-docs On Mon, Apr 11, 2005 at 22:34:21 -0600, Michael Fuhr <[email protected]> wrote: > On Mon, Apr 11, 2005 at 11:40:01PM -0500, Bruno Wolff III wrote: > > I was looking at: > > http://candle.pha.pa.us/main/writings/pgsql/sgml/performance-tips.html > > > > And noticed in the first example that it is claimed the cost estimate > > is 233 disk block reads. However, "233" doesn't appear in the explain > > output shown. I think that "233" is supposed to match the "333" in > > the explain output, but I am not 100% sure. > > This part? > > This is about as straightforward as it gets. If you do > > SELECT * FROM pg_class WHERE relname = 'tenk1'; > > you will find out that tenk1 has 233 disk pages and 10000 rows. > So the cost is estimated at 233 page reads, defined as costing > 1.0 apiece, plus 10000 * cpu_tuple_cost which is currently 0.01 > (try SHOW cpu_tuple_cost). > > Doesn't that work out to > > (233 * 1.0) + (10000 * 0.01) = 233.0 + 100.0 = 333.0 > > or am I missing something? No, I was. I misread a comma as a period and when I scanned ahead to see if I could find something similar in other examples I missed the part where it added in the cpu cost. After I sent the message I looked some more and spotted what I missed. I didn't have a copy back yet of the original message, so my oops message wasn't threaded with the original. ^ permalink raw reply [nested|flat] 3+ messages in thread
end of thread, other threads:[~2005-04-12 05:01 UTC | newest] Thread overview: 3+ messages (download: mbox mbox.gz follow: Atom feed) -- links below jump to the message on this page -- 2005-04-12 04:40 Possible copy error in explain example Bruno Wolff III <[email protected]> 2005-04-12 04:34 ` Michael Fuhr <[email protected]> 2005-04-12 05:01 ` Bruno Wolff III <[email protected]>
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