Re: Curious about dead rows.

Tom Lane wrote:
> Russell Smith <mr-russ(at)pws(dot)com(dot)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.

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