Tom Lane wrote:
> Russell Smith <mr-russ@pws.com.au> writes:
>> It is possible that analyze is not getting the number of dead rows
>> right?
>
> Hah, I think you are on to something. ANALYZE is telling the truth about
> how many "dead" rows it saw, but its notion of "dead" is "not good
> according to SnapshotNow". Thus, rows inserted by a not-yet-committed
> transaction would be counted as dead. So if these are background
> auto-analyzes being done in parallel with inserting transactions that run
> for awhile,
They are.
> seeing a few not-yet-committed rows would be unsurprising.
That is a very interesting possibility. I can see that it is certainly a
possible explanation, since my insert transactions take between 0.04 to 0.1
minutes (sorry, decimal stopwatch) of real time, typically putting 1700 rows
into about a half dozen tables. And the ANALYZE is whatever autovacuum
chooses to do. So if new not-yet-committed rows are considered dead, that
would be a sufficient explanation.
So I am, retroactively, unsurprised.
> I wonder if that is worth fixing? I'm not especially concerned about the
> cosmetic aspect of it, but if we mistakenly launch an autovacuum on the
> strength of an inflated estimate of dead rows, that could be costly.
>
Well, since I was more interested in the explanation than in the fixing, in
that sense I do not care if it is fixed or not. While it may create a slight
slowdown (if it is an error), the applications run "fast enough."
I would not even get the fix until Red Hat get around to putting it in (I
run postgresql-8.1.9-1.el5 that is in their RHEL5 distribution), that
probably will not be until RHEL6 and the soonest, and I will probably skip
that one and wait until RHEL7 comes out in about 3 years.
But somewhere perhaps a reminder of this should be placed where someone like
me would find it, so we would not have to go through this again for someone
else.
--
.~. Jean-David Beyer Registered Linux User 85642.
/V\ PGP-Key: 9A2FC99A Registered Machine 241939.
/( )\ Shrewsbury, New Jersey http://counter.li.org
^^-^^ 22:05:01 up 22 days, 15:23, 0 users, load average: 4.16, 4.22, 4.10
