Improve the accuracy of sample stats collection. For non distribution columns, use
`n*d / (n - f1 + f1*n/N)`
where `f1` is the number of distinct values that occurred exactly once in our sample of n rows (from a total of N),
and `d` is the total number of distinct values in the sample.
For distribution columns, use `ndv(n) * fraction of tablets sampled` for NDV.
For very large tablet to sample, use limit to control the total lines to scan (for non key column only, because key column is sorted and will be inaccurate using limit).
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