RE: Histograms - SIZE clause & num_buckets anomaly
- From: "Charudatta Joshi" <joshic@xxxxxxxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Tue, 3 Aug 2004 16:57:44 +0530
Hi Lex,
Been thinking about this comment of yours:
>>I agree that frequency histograms give the ultimate information about a
>>column population, compared with height-based histograms, but on the other
>>hand the performance should be "comparable" -- that is, the height-based
>>histogram with a reasonably high number of buckets shouldn't give bad
>>execution plans.
Currently, wherever possible & applicable, I (blindly) have the number of
buckets equal to the number of distinct values. This means that whenever
number of distinct values is less than 255, that is my bucket size.
Given the fact that histogram maintenance can be very expensive, I now
wonder if I should try to get the same benefit using less number of buckets.
I guess in this case the logic for determining the bucket size will vary
from case to case. But in general:
-- Should one experiment with different histogram sizes even if the NDV is
very less?
-- Roughly what difference in the number of buckets justifies using h.b.
over f.b.? For e.g. if I have to use 100 buckets for f.b. and 85 for h.b.,
that doesn't seem to be a good bargain for h.b.
I realize the answers can vary from case to case, still a general opinion
will be much appreciated.
Thanks & regards,
Charu.
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