Jim C. Nasby wrote:
>Ahh, I was thinking of a high correlation factor on the index. I still
>question 5% though... that seems awefully low.
>
>
Not really. It all depends on how many records you're packing into each
page. 1% may well be the threshold for small records.
Tom mentioned this in the last couple of months. He was citing a uniform
distribution as an example and I thought that sounded a little
pessimistic, but when I did the (possibly faulty) math with a random
distribution, I discovered he wasn't far off.
It's not this simple, but if you can fit 50 randomly organized records
into each page and you want to retrieve 2% of the rows, it's likely
you'll have to fetch every page - believe it or not.
What concerns me is that this all depends on the correlation factor, and
I suspect that the planner is not giving enough weight to this.
Actually, I'm wondering if it's even looking at the statistic, but I
haven't created a test to check. It might explain quite a few complaints
about the planner not utilizing indexes.
