From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Greg Stark <stark(at)mit(dot)edu> |
Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, "<pgsql-hackers(at)postgresql(dot)org>" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Some belated patch review for "Buffers" explain analyze patch |
Date: | 2010-02-10 01:18:41 |
Message-ID: | 1945.1265764721@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Greg Stark <stark(at)mit(dot)edu> writes:
> On Wed, Feb 10, 2010 at 12:32 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> The reason that EXPLAIN prints things the way it does is so that actual
>> costs/times are comparable to estimated costs.
> Oh, that was a thought I had along the way but forgot to mention in my
> email: since the buffer usage isn't related to the cost there isn't
> nearly the impetus to divide by loops except to be consistent with the
> time.
True. BTW I realized that I was confusing the buffer-usage issue with
the recent changes to add printing of hash join execution stats.
I believe offhand that for both that and the Sort statistics, what
you see is really just stats for the *last* execution of the node,
if it got executed more than once. While that's incomplete, at least
it is somewhat consistent --- you won't see numbers that are stated
on a completely different basis from those around them.
We'd have to think about whether and how to adjust the sort and hashjoin
stats if we wanted to switch over to whole-query totals.
> Perhaps instead of looking to change the "actual" times we should look
> at a way to include total time spent in each node.
You can already get that by multiplying the displayed total time by the
number of loops. Robert does have a point that this is subject to a lot
of roundoff error, though, when the per-loop time is much less than 1
msec. I wouldn't object to adding a "total time" field to the
machine-readable formats.
regards, tom lane
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