Re: [PATCHES] Proposed patch: synchronized_scanningGUCvariable

Jeff Davis wrote:
> On Mon, 2008-01-28 at 23:13 +0000, Heikki Linnakangas wrote:
>> It's a good point that we don't want pg_dump to screw up the cluster
>> order, but that's the only use case I've seen this far for disabling
>> sync scans. Even that wouldn't matter much if our estimate for
>> "clusteredness" didn't get screwed up by a table that looks like this:
>> "5 6 7 8 9 1 2 3 4"
>
> It doesn't seem like there is any reason for the estimate to get
> confused, but it apparently does. I loaded a test table with a similar
> distribution to your example, and it shows a correlation of about -0.5,
> but it should be as good as something near -1 or +1.
>
> I am not a statistics expert, but it seems like a better measurement
> would be: "what is the chance that, if the tuples are close together in
> index order, the corresponding heap tuples are close together?".
>
> The answer to that question in your example is "very likely", so there
> would be no problem.
>
> Is there a reason we don't do this?

It has been discussed before, but no-one has come up with a good
measurement for that.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com