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Re: Autovacuum versus rolled-back transactions

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Autovacuum versus rolled-back transactions
Date: 2007-06-01 03:22:40
Message-ID: 16039.1180668160@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> Tom Lane wrote:
>> Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
>>> If we apply Heikki's idea of advancing OldestXmin, I think what we
>>> should do is grab the value from pgstats when vacuum starts, and each
>>> time we're going to advance OldestXmin, grab the value from pgstats

>> Considering that each of those values will be up to half a second old,
>> I can hardly think that this will accomplish anything except to
>> introduce a great deal of noise ...

> Normally, yes, but the values can be older if the vacuum_cost_delay is
> large.

I'm not sure we're on the same page.  I meant that whatever you read
from pgstats is going to be stale by an uncertain amount of time.
Taking the deltas of such numbers over relatively short intervals
is going to be mighty noisy.

			regards, tom lane

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