Robert Haas <robertmhaas@gmail.com> writes:
> The reason I thought cross-column correlations might be relevant is
> that the bitmap index scan on news_visible_from is quite accurate
> (19976 estimated vs. 19932 actual) and the bitmap index scan on
> news_visible_to is tolerably accurate (151 estimated vs. 41 actual)
> but the estimate on the BitmapOr is somehow totally wrong (20127
> estimated vs. 0 actual). But on further reflection that doesn't make
> much sense. How can the BitmapOr produce fewer rows than the sum of
> its constituent inputs?
That's not an estimation bug, that's a measurement bug. We don't try to
count the actual number of rows present in the result of a BitmapOr or
BitmapAnd node. (It would be impractical in lossy cases anyway, not to
mention expensive.)
regards, tom lane