> No, it's certainly not the right thing. To my understanding, disbursion
> is a measure of the frequency of the most common value of an attribute;
> but that tells you very little about how many other values there are.
> 1/disbursion is a lower bound on the number of values, but it wouldn't
> be a good estimate unless you had reason to think that the values were
> pretty evenly distributed. There could be a *lot* of very-infrequent
> values.
>
> > with 100 distinct values of an attribute uniformly distribuited in a
> > relation of 10000 tuples, disbursion was estimated as 0.002275, giving
> > us 440 distinct values.
>
> This is an illustration of the fact that Postgres' disbursion-estimator
> is pretty bad :-(. It usually underestimates the frequency of the most
> common value, unless the most common value is really frequent
> (probability > 0.2 or so). I've been trying to think of a more accurate
> way of figuring the statistic that wouldn't be unreasonably slow.
> Or, perhaps, we should forget all about disbursion and adopt some other
> statistic(s).
Yes, you have the crux of the issue. I wrote it because it was the best
thing I could think of, but it is non-optimimal. Because all the
optimal solutions seemed too slow to me, I couldn't think of a better
one.
Here is my narrative on it from vacuum.c:
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* We compute the column min, max, null and non-null counts.* Plus we attempt to find the count of the value that
occursmost* frequently in each column* These figures are used to compute the selectivity of the column** We use a
three-buckedcache to get the most frequent item* The 'guess' buckets count hits. A cache miss causes guess1* to get
themost hit 'guess' item in the most recent cycle, and* the new item goes into guess2. Whenever the total count of
hits* of a 'guess' entry is larger than 'best', 'guess' becomes 'best'.** This method works perfectly for columns with
uniquevalues, and columns* with only two unique values, plus nulls.** It becomes less perfect as the number of unique
valuesincreases and* their distribution in the table becomes more random.
-- Bruce Momjian | http://www.op.net/~candle maillist@candle.pha.pa.us | (610)
853-3000+ If your life is a hard drive, | 830 Blythe Avenue + Christ can be your backup. | Drexel Hill,
Pennsylvania19026