From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
---|---|
To: | Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov> |
Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Florian Helmberger <fh(at)25th-floor(dot)com>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Greg Stark <gsstark(at)mit(dot)edu>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: [ADMIN] pg_class reltuples/relpages not updated by autovacuum/vacuum |
Date: | 2011-05-26 17:50:45 |
Message-ID: | BANLkTikTbZ8gibj+X++vkdqkxxJCU6trWw@mail.gmail.com |
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On Thu, May 26, 2011 at 1:28 PM, Kevin Grittner
<Kevin(dot)Grittner(at)wicourts(dot)gov> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> I think we should really consider replacing reltuples with
>> reltupledensity at some point. I continue to be afraid that using
>> a decaying average in this case is going to end up overweighting
>> the values from some portion of the table that's getting scanned
>> repeatedly, at the expense of other portions of the table that are
>> not getting scanned at all. Now, perhaps experience will prove
>> that's not a problem. But storing relpages and reltupledensity
>> separately would give us more flexibility, because we could feel
>> free to bump relpages even when we're not sure what to do about
>> reltupledensity. That would help Florian's problem quite a lot,
>> even if we did nothing else.
>
> Given how trivial it would be to adjust reltuples to keep its ratio
> to relpages about the same when we don't have a new "hard" number,
> but some evidence that we should fudge our previous value, I don't
> see where this change buys us much. It seems to mostly obfuscate
> the fact that we're changing our assumption about how many tuples we
> have. I would rather that we did that explicitly with code comments
> about why it's justified than to slip it in the way you suggest.
I'm a bit confused by this - what the current design obfuscates is the
fact that reltuples and relpages are not really independent columns;
you can't update one without updating the other, unless you want
screwy behavior. Replacing reltuples by reltupledensity would fix
that problem - it would be logical and non-damaging to update either
column independently.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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