On 2019-09-12 21:04:25 +0200, Peter J. Holzer wrote: > On 2019-09-12 12:54:55 -0400, Tom Lane wrote: > > It's not taking the partial-index filter into account in that, I > > suspect, which skews the results in this case --- but that would be > > hard to account for accurately. > > Hmm. Wouldn't that be a problem for partial indexes in general? They > usually cover only a small portion of the table and if the selectivity > is computed relative to the whole table the result may be way off. > > I think it should be possible to adjust for a "WHERE column IS NOT NULL" > filter, because null_frac is in the statistics. For the general case you > would need an estimate of the number of rows covered by the index, which > I don't think we have.
Looking through the source I see that the planner does estimate the
number of tuples in the index.
hp
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