be4b4dc759
Previously, we included null rows in the values of n and N that went into the formula, which amounts to considering null as a value in its own right; but the d and f1 values do not include nulls. This is inconsistent, and it contributes to significant underestimation of ndistinct when the column is mostly nulls. In any case stadistinct is defined as the number of distinct non-null values, so we should exclude nulls when doing this computation. This is an aboriginal bug in our application of the Haas-Stokes formula, but we'll refrain from back-patching for fear of destabilizing plan choices in released branches. While at it, make the code a bit more readable by omitting unnecessary casts and intermediate variables. Observation and original patch by Tomas Vondra, adjusted to fix both uses of the formula by Alex Shulgin, cosmetic improvements by me
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