Re: [PATCHES] Better default_statistics_target

On Mon, Jan 28, 2008 at 11:14:05PM +0000, Christopher Browne wrote:
> On Dec 6, 2007 6:28 PM, Decibel! <decibel(at)decibel(dot)org> wrote:
> > FWIW, I've never seen anything but a performance increase or no change
> > when going from 10 to 100. In most cases there's a noticeable
> > improvement since it's common to have over 100k rows in a table, and
> > there's just no way to capture any kind of a real picture of that with
> > only 10 buckets.
>
> I'd be more inclined to try to do something that was at least somewhat
> data aware.
>
> The "interesting theory" that I'd like to verify if I had a chance
> would be to run through a by-column tuning using a set of heuristics.
> My "first order approximation" would be:
>
> - If a column defines a unique key, then we know there will be no
> clustering of values, so no need to increase the count...
>
> - If a column contains a datestamp, then the distribution of values is
> likely to be temporal, so no need to increase the count...
>
> - If a column has a highly constricted set of values (e.g. - boolean),
> then we might *decrease* the count.
>
> - We might run a query that runs across the table, looking at
> frequencies of values, and if it finds a lot of repeated values, we'd
> increase the count.
>
> That's a bit "hand-wavy," but that could lead to both increases and
> decreases in the histogram sizes. Given that, we can expect the
> overall stat sizes to not forcibly need to grow *enormously*, because
> we can hope for there to be cases of shrinkage.

I think that before doing any of that you'd be much better off
investigating how much performance penalty there is for maxing out
default_statistict_target. If, as I suspect, it's essentially 0 on
modern hardware, then I don't think it's worth any more effort.

BTW, that investigation wouldn't just be academic either; if we could
convince ourselves that there normally wasn't any cost associated with a
high default_statistics_target, we could increase the default, which
would reduce the amount of traffic we'd see on -performance about bad
query plans.
--
Decibel!, aka Jim C. Nasby, Database Architect decibel(at)decibel(dot)org
Give your computer some brain candy! www.distributed.net Team #1828