Message-ID: <51916C59.6010700@catalyst.net.nz>
Date: Tue, 14 May 2013 10:42:33 +1200
From: Mark Kirkwood <mark.kirkwood@catalyst.net.nz>
User-Agent: Mozilla/5.0 (X11; Linux x86_64;
	rv:17.0) Gecko/20130404 Thunderbird/17.0.5
MIME-Version: 1.0
To: Marti Raudsepp <marti@juffo.org>, ach <alanchines@gmail.com>
CC: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: statistics target for columns in unique constraint?
References: <1368457293547-5755256.post@n5.nabble.com>
	<CABRT9RBwmDxjq-tzLMh6rFX-suJYVQMiGSQAADsueg07rvB1dg@mail.gmail.com>
In-Reply-To: 
 <CABRT9RBwmDxjq-tzLMh6rFX-suJYVQMiGSQAADsueg07rvB1dg@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk
Sender: pgsql-performance-owner@postgresql.org

On 14/05/13 10:10, Marti Raudsepp wrote:
> On Mon, May 13, 2013 at 6:01 PM, ach <alanchines@gmail.com> wrote:
>> what I'm wondering is, since
>> the unique constraint already covers the whole table and all rows in
>> entirety, is it really necessary for statistics to be set that high on
>> those?
>
> AFAIK if there are exact-matching unique constraints/indexes for a
> query's WHERE clause, the planner will deduce that the query only
> returns 1 row and won't consult statistics at all.
>
>> Or does that only serve to slow down inserts to that table?
>
> It doesn't slow down inserts directly. Tables are analyzed in the
> background by autovacuum. However, I/O traffic from autovacuum analyze
> may slow down inserts running concurrently.
>
>

A higher number in stats target means larger stats structures - which in 
turn means that the planning stage of *all* queries may be impacted - 
e.g takes up more memory, slightly slower as these larger structures are 
read, iterated over, free'd etc.

So if your only access is via a defined unique key, then (as Marti 
suggests) - a large setting for stats target would seem to be unnecessary.

If you have access to a test environment I'd recommend you model the 
effect of reducing stats target down (back to the default of 100 or even 
to the old version default of 10).

A little - paranoia - maybe switch on statement logging and ensure that 
there are no *other* ways this table is accessed...the fact that the 
number was cranked up from the default is a little suspicious!

Regards

Mark


-- 
Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-performance