> I don't see why the MCVs would need a particularly large sample size
> to calculate accurately. Have you done any tests on the accuracy of
> the MCV list?
Yes, although I don't have them at my fingertips. In sum, though, you
can't take 10,000 samples from a 1b row table and expect to get a
remotely accurate MCV list.
A while back I did a fair bit of reading on ndistinct and large tables
from the academic literature. The consensus of many papers was that it
took a sample of at least 3% (or 5% for block-based) of the table in
order to have 95% confidence in ndistinct of 3X. I can't imagine that
MCV is easier than this.
> And mostly
> what it tells me is that we need a robust statistical method and the
> data structures it requires for estimating the frequency of a single
> value.
Agreed.
> Binding the length of the MCV list to the size of the histogram is
> arbitrary but so would any other value and I haven't seen anyone
> propose any rationale for any particular value.
histogram size != sample size. It is in our code, but that's a bug and
not a feature.
--
-- Josh Berkus
PostgreSQL Experts Inc.
http://www.pgexperts.com
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