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Re: Clearing global statistics

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Greg Smith <greg(at)2ndquadrant(dot)com>, Rafael Martinez <r(dot)m(dot)guerrero(at)usit(dot)uio(dot)no>, Magnus Hagander <magnus(at)hagander(dot)net>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Clearing global statistics
Date: 2010-01-17 00:33:13
Message-ID: 603c8f071001161633m27d80fbdje1c5cf28ae72c855@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Thu, Jan 14, 2010 at 1:11 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Greg Smith <greg(at)2ndquadrant(dot)com> writes:
>> Tom Lane wrote:
>>> Actually, that brings up a more general question: what's with the
>>> enthusiasm for clearing statistics *at all*?
>
>> ... Right now, you're still carrying around
>> the history of the bad period forever though, and every check of the
>> pg_stat_bgwriter requires manually subtracting the earlier values out.
>
> Seems like a more appropriate solution would be to make it easier to do
> that subtraction, ie, make it easier to capture the values at a given
> time point and then get deltas from there.  It's more general (you could
> have multiple saved sets of values), and doesn't require superuser
> permissions to do, and doesn't have the same potential for
> damn-I-wish-I-hadn't-done-that moments.

True, but it's also more complicated to use.  Most systems I'm
familiar with[1] that have performance counters just provide an option
to clear them.  Despite the disadvantages you cite, it seems to be
fairly useful in practice; anyway, I have found it so.

...Robert

[1] The other design I've seen is a system that automatically resets,
say, once a day.  It retains the statistics for the 24-hour period
between the most recent two resets, and the statistics for the partial
period following the last reset.  But that doesn't seem appropriate
for PostgreSQL....

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