Jim C. Nasby wrote:
Ahh, I was thinking of a high correlation factor on the index. I stillNot really. It all depends on how many records you're packing into each page. 1% may well be the threshold for small records.
question 5% though... that seems awefully low.
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.
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